Best Oil for Diesel Trucks in Cold Climates Best Oil for Diesel Trucks in Cold Climates

Winter diesel viscosity and cold-start guide

Choose Diesel Oil That Pumps Quickly Without Losing Required Protection

The best oil for a diesel truck in cold weather is the lowest winter-grade viscosity that your engine manufacturer approves for the temperatures expected during the entire service interval. For many modern pickups, that may be an approved synthetic 5W-40 or 0W-40—but neither grade is universal.

This guide helps you compare 0W-40, 5W-40, 10W-30 and 15W-40, verify API, ACEA and manufacturer approvals, prepare for sub-zero starts and avoid confusing engine-oil problems with diesel-fuel gelling or weak batteries.

First priority Exact OEM viscosity and approval

Common cold choice Approved full-synthetic 5W-40

Extreme-cold option Approved 0W-grade only when listed

Do not assume Every CK-4 or 5W-40 oil fits every diesel

Choose the right winter oil Compare viscosity grades Prepare for cold starts

Check the lowest expected temperature Choose for the coldest realistic overnight temperature before the next oil change—not today’s afternoon temperature.

Match the complete specification Confirm viscosity, API or ACEA category and every required manufacturer approval on the oil label or data sheet.

Prepare the entire starting system Correct oil cannot compensate for weak batteries, gelled fuel, failed glow plugs or an unsuitable coolant mixture.

No universal bottle fits every diesel

How to Choose Diesel Engine Oil for Cold Weather

Identify the exact truck and engine. Write down the model year, make, model, engine displacement, engine code and emissions configuration. Use the VIN when the model has more than one diesel engine or a mid-year specification change.

Find the official engine-oil specification. Use the owner’s manual or engine manufacturer’s service information. Record the permitted SAE grades, API or ACEA category, OEM approval, oil capacity and severe-service notes.

Choose for the coldest temperature expected before the next change. If the truck will travel from a mild region into arctic conditions, use the lowest temperature expected during that trip and service interval.

Verify the winter grade is actually approved. A 0W or 5W oil may flow better when cold, but it must still meet the engine’s hot-viscosity and performance requirements.

Read the back label or current product data sheet. Confirm the exact API category, ACEA sequence and OEM approval. Do not rely only on words such as “heavy duty,” “fleet,” “turbo diesel” or “extreme protection.”

Use a filter suitable for the full service interval. Match the exact engine application and replace the filter according to the truck or engine manufacturer’s schedule.

Change to the winter oil before temperatures fall. Do not wait until the sump contains thick summer-grade oil and the truck is already parked outdoors in extreme cold.

Fast manual search: Search the digital owner’s manual for “engine oil,” “viscosity,” “cold weather,” “ambient temperature,” “block heater,” “severe service” and “capacities.”

The oil brand is not the specification. A manufacturer may sell several diesel oils with different viscosity grades and approvals. Match the exact product name and specification—not merely the logo on the bottle.

Practical winter recommendation

What Is the Best Diesel Oil for a Cold Climate?

Modern diesel pickup below 0°F / −18°C An OEM-approved full-synthetic 5W-40 is a common recommendation for some Cummins/Ram applications, but verify the exact engine and model year.Extreme cold near −22°F / −30°C or lower Some current Ford diesel guidance permits an approved 0W-40 for improved cold-start performance. Do not use it in another engine unless that manufacturer also approves it.Moderately cold winter An approved 10W-30 may remain correct for certain engines and temperature ranges. Do not replace it automatically with 5W-40 based only on internet advice.Older engine using 15W-40 Use 15W-40 only within the manufacturer’s permitted temperature range. Many trucks require or benefit from an approved lower-W grade in colder conditions.European or international heavy-duty truck Match the required ACEA sequence and engine-maker approval in addition to viscosity. API CK-4 alone may not satisfy the manufacturer’s requirement.Modified or rebuilt diesel Use the engine builder’s recommendation based on bearing clearances, oil pressure, turbocharger and intended winter temperature.

Best simple rule: Select the lowest “W” grade explicitly approved for your engine and expected temperature while keeping the required second number and performance approvals.

Understand what the numbers mean

0W-40 vs 5W-40 vs 10W-30 vs 15W-40

The first number and “W” describe low-temperature cranking and pumping performance under defined laboratory tests. The second number describes the oil’s viscosity grade at operating temperature. These numbers do not measure oil quality or prove vehicle compatibility.

Grade	Cold-climate strength	Typical use	Important limitation
0W-40	Strong low-temperature cranking and pumpability compared with higher-W grades	Extreme cold where the engine manufacturer lists an approved 0W-40	Not automatically approved merely because the engine normally uses a 40-grade oil
5W-40	Practical cold-weather flow with a 40 operating-temperature grade	Common synthetic winter or severe-duty choice for some modern diesel pickups	Must meet the required API, ACEA and OEM specifications
10W-30	Can work well in moderate cold when specifically approved	Factory recommendation for several modern diesel engines above stated temperature limits	May not be the manufacturer’s choice for very low or arctic temperatures
15W-40	Generally less suitable for very cold cranking than approved 0W, 5W or 10W grades	Warm-weather, legacy and heavy-duty applications where the manual permits it	Do not use below the listed temperature range simply because it is traditional diesel oil
5W-30 or 0W-30	Good cold flow where the engine is designed for an XW-30 grade	Selected light-duty, newer on-highway or European diesel engines	Must not replace an approved 40-grade unless the manufacturer permits it

Do not create your own viscosity blend. Mixing 0W-40 and 15W-40 does not produce a certified intermediate winter grade. The final mixture has not been tested or licensed as a specific SAE viscosity.

Use temperature as a lookup trigger

Cold-Weather Oil Decision by Temperature

32°F / 0°C Cold-start quality begins to matter more. Confirm batteries, fuel and the manual’s winter viscosity.

0°F / −18°C Some Cummins/Ram guidance changes from approved 10W-30 to synthetic 5W-40 below this point.

−10°F / −23°C Several Ford diesel manuals recommend use of an engine block heater at approximately this temperature.

−22°F / −30°C Current Ford guidance identifies approved 0W-40 as an extreme-cold alternative for certain 6.7L applications.

These are examples, not universal changeover temperatures. The engine manufacturer’s chart for your exact model year and engine controls the decision.

Official examples show why exact lookup matters

Cold-Climate Oil Examples for Popular Diesel Trucks

Application example	Published cold-weather guidance	Required specification	What this teaches
2021 Cummins 6.7L pickup application	10W-30 above 0°F / −18°C; synthetic 5W-40 below 0°F / −18°C	API CK-4 plus manufacturer Material Standard MS-10902; qualifying CES 20081 products may also be used	The recommended grade can change at a specific temperature.
2022 Ram 6.7L Cummins	10W-30 above 0°F / −18°C; synthetic 5W-40 below 0°F / −18°C	API CK-4 and MS-10902; qualifying CES 20081 products may also be used	Viscosity and material standard must both match.
Current Ford 6.7L Super Duty guidance	10W-30 standard listing, 5W-40 for severe duty and approved 0W-40 when ambient temperature reaches −22°F / −30°C or below	Ford WSS-M2C171-F1	A 0W-grade can be an official extreme-cold alternative, but only with the required Ford specification.
Older Ford 6.7L applications	Specifications and API categories can differ by model year	Use the exact manual rather than applying current guidance backward automatically	Model year matters even when the engine name is unchanged.

Do not use this comparison table as a substitute for your manual. Capacities, approved categories and viscosity charts can change by model year, market, emissions package and engine revision.

Cummins example: Check the published 6.7L guidance for oil-level procedure, severe-service conditions, temperature-based viscosity and required API/OEM standards.

Review official Cummins 6.7L oil guidance

The letters matter as much as the viscosity

API CK-4, FA-4, ACEA and OEM Diesel Oil Approvals

Common North American category

API CK-4

CK-4 is a current heavy-duty diesel category designed for modern high-speed diesel engines and aftertreatment systems. It also addresses oxidation, shear, aeration, wear, deposits and soot-related viscosity increase.

Selected engines only

API FA-4

FA-4 covers certain lower-HTHS XW-30 oils for selected newer on-highway diesel engines. It is not backward compatible or interchangeable with CK-4 unless the engine manufacturer specifically permits it.

International heavy-duty use

ACEA and OEM claims

European and global trucks may require ACEA E or F sequences plus an engine-maker approval from companies such as Mercedes-Benz, Volvo, Mack, MAN, Scania or Renault Trucks.

SAE viscosity Shows cold and operating-temperature viscosity grade, such as 5W-40.API category Shows a North American diesel performance category, such as CK-4 or FA-4.ACEA sequence Shows an applicable European performance sequence for light- or heavy-duty engines.OEM approval Shows that the product meets a specific engine or vehicle manufacturer requirement.Product data sheet Provides detailed claims, test properties and approvals that may not fit on the bottle label.

Never substitute FA-4 for CK-4 by viscosity alone. A CK-4 10W-30 and an FA-4 10W-30 can share the same SAE grade while being intended for different engine requirements.

API label check: Look for the API service symbol, read the category in the top portion and confirm the SAE viscosity in the center. Then compare both with the engine manual.

Review official API diesel oil categories

International trucks need local specifications

Cold-Climate Diesel Oil Outside North America

Truck model names, engine calibrations, fuels and oil approvals can differ by country. An imported pickup may not match the oil listing for a locally sold version with the same badge.

Europe

Check the required ACEA sequence and manufacturer approval. Low-ash compatibility may be essential for DPF-equipped engines.

Canada and arctic regions

Use the Canadian or regional owner’s manual, winterized fuel, correct block-heater procedure and a battery setup rated for expected temperatures.

India and mountain regions

Use the engine maker’s local viscosity chart for Himalayan or high-altitude winter operation. Do not apply lowland summer specifications automatically.

Australia and New Zealand

Some inland or alpine areas can experience freezing conditions even when the national climate is generally mild. Choose for the route and overnight location.

Imported Japanese diesels

Match the exact engine code, DPF requirement and regional manual. Grey-import specifications can differ from domestic-market listings.

Commercial fleets

Use engine-builder approvals, ACEA/API requirements, engine hours, oil analysis and the fleet’s lowest operating temperature.

ACEA categories do not choose viscosity for you. ACEA’s heavy-duty sequences allow manufacturers to specify viscosity according to engine design and ambient temperature.

International standard reference: ACEA’s 2024 heavy-duty oil sequences include current E and F categories, while individual vehicle manufacturers may impose additional viscosity and approval requirements.

Review the 2024 ACEA heavy-duty oil sequences

Why synthetic is often practical in winter

Is Full-Synthetic Diesel Oil Better in Cold Weather?

Full-synthetic oil is often the most practical way to achieve approved 0W and 5W diesel grades with strong low-temperature cranking, pumpability and high-temperature stability. However, “full synthetic” does not prove the oil meets your engine’s specification.

Cold cranking

An approved lower-W synthetic can reduce resistance during starting compared with a higher-W grade allowed only for warmer conditions.

Oil pump supply

Correct winter viscosity helps oil reach bearings, camshafts, valve-train parts and turbocharger passages after startup.

Temperature range

A quality synthetic can provide cold-flow performance while maintaining the required operating-temperature grade.

Synthetic oil does not automatically extend the service interval. Follow the oil-life monitor or the manufacturer’s mileage, time and engine-hour limit—especially with idling, towing, short trips or regeneration-heavy use.

Oil is only one part of cold starting

When to Use a Diesel Engine Block Heater

A block heater warms part of the engine or coolant system before startup. It can reduce cold-start strain, improve cabin warm-up and help the engine reach stable operation sooner. Use only the installed heater and procedure approved for the truck.

Check the manufacturer’s temperature recommendation. Several Ford diesel manuals recommend block-heater use at approximately −10°F / −23°C or below, but the threshold can vary.

Inspect the cord and connector before winter. Do not use a cracked, burnt, wet or damaged cord.

Use a correctly rated outdoor extension cord when permitted. Follow electrical codes and the owner’s manual. Avoid light indoor cords and damaged outlets.

Plug in for the recommended period. Longer is not always more useful. A suitable timer can limit unnecessary electricity use where the manual permits it.

Unplug before moving the truck. Make the cord part of the walk-around inspection so it is not torn from the vehicle or outlet.

Do not use external flame or improvised heat. Open flames, charcoal, space heaters and unapproved heating methods create fire, exhaust and component-damage risks.

Block-heater check: If the truck remains difficult to start despite being plugged in, verify that the outlet has power and have the heater circuit tested. Do not assume the heater is working merely because the cord is connected.

Protect the engine during the first minutes

How to Start a Diesel Truck Safely in Extreme Cold

Turn off unnecessary electrical loads. Reduce demand from lights, seat heaters, blowers and accessories before cranking when the owner’s manual recommends it.

Allow the glow-plug or intake-heater cycle to complete. Wait for the dashboard message or indicator. Do not repeatedly cycle the system unless the manufacturer instructs you to do so.

Do not press the accelerator during starting unless instructed. Modern diesel engine controls normally manage starting fuel and air automatically.

Release the starter within the specified limit. Do not crank continuously until the batteries and starter overheat. Follow the manual’s rest period between attempts.

Watch the oil-pressure warning. It should go out promptly. Shut the engine off if the red warning remains on or abnormal mechanical noise continues.

Allow a brief stabilization period. Some manufacturers specify several minutes of idle in extreme cold or use an automatic warm-up strategy.

Drive gently rather than idling indefinitely. Use light load and moderate engine speed until oil, coolant, transmission and axle lubricants begin warming.

Avoid immediate heavy towing or full throttle. A running engine is not necessarily a fully lubricated and warmed drivetrain.

Do not use starting fluid unless the engine manufacturer explicitly allows it. Ether can ignite against glow plugs or intake heaters and cause personal injury or severe engine damage.

A no-start may not be caused by oil

Cold Oil vs Gelled Diesel Fuel vs Weak Batteries

What the truck does	Possible cause	Practical next check
Cranks very slowly	Weak batteries, poor cable connection, starter problem or oil too viscous for the conditions	Test both batteries where fitted, inspect connections and verify the approved winter oil.
Cranks normally but does not fire	Glow-plug/intake-heater fault, fuel issue, low compression, sensor or control fault	Check warnings, diagnostic codes and the cold-start system.
Starts, stalls and will not restart	Fuel waxing or restricted fuel filters can starve the engine	Move to warmth when possible and follow the fuel-system cold-weather procedure.
White smoke and rough running after startup	Cold combustion, unsuitable fuel, failed glow system or injector issue	Allow approved warm-up and diagnose symptoms that persist.
Red oil-pressure warning stays on	Pressure failure, low level, wrong oil, blocked pickup or mechanical fault	Shut the engine off immediately. Do not continue warming it.
Normal starting but no power in deep cold	Fuel restriction, frozen moisture, charge-air issue or protection strategy	Check the fuel/water separator, filters, warnings and manufacturer guidance.

Engine oil does not prevent diesel fuel from gelling. Winterized diesel, suitable anti-gel treatment when approved, maintained fuel filters and water-separator service are separate requirements.

Cold-weather diesel fuel preparation

Winter Fuel Matters as Much as Winter Oil

Use seasonally suitable diesel

Buy from a busy, reputable fuel station.
Use winterized or climatized diesel where available.
Know whether the truck permits biodiesel in extreme cold.
Do not blend gasoline or alcohol into diesel.
Use only manufacturer-approved anti-gel products and dosage.

Maintain the fuel system

Replace fuel filters on schedule.
Drain the fuel/water separator as instructed.
Keep the tank adequately filled to reduce condensation risk.
Carry spare filters only when you know the safe replacement procedure.
Investigate recurring water-in-fuel warnings.

Do not pour anti-gel into already waxed fuel and expect an immediate roadside repair. Many preventive products must be mixed before fuel reaches its cloud or pour point. Follow the product and vehicle instructions.

Ram cold-fuel example: Official guidance explains use of climatized diesel, Number 1 diesel in appropriate arctic conditions and approved treatment when suitable winter fuel is unavailable.

Review official Ram diesel fuel requirements

Diesel starting requires electrical power

Battery and Starting-System Checklist

Before winter

Load-test every battery in a dual-battery system.
Verify both batteries are compatible in age, capacity and condition.
Clean and tighten battery terminals and grounds.
Inspect starter cables for corrosion and heat damage.
Confirm alternator output and charging voltage.

Before a severe cold start

Turn off avoidable electrical accessories.
Confirm the transmission is in Park or Neutral.
Allow the glow system to complete its cycle.
Avoid repeated long cranking attempts.
Use only the manufacturer’s approved jump-start points and procedure.

A diesel with two batteries can still crank with one failing battery. The weak unit can overload the healthy battery and create inconsistent cold-start performance. Test the pair as a system.

Winter can qualify as severe operation

Oil-Change Intervals in Cold Climates

Cold weather does not create one universal shorter interval, but repeated cold starts, short trips, long idling, snow-plow use and regeneration-heavy operation can place the truck in a severe-service category.

Short journeys

The oil may not stay hot long enough to evaporate moisture and fuel contamination.

Extended idling

Engine hours accumulate while odometer mileage remains low. Track hours where the truck provides them.

Snow-plow operation

Low-speed load, repeated direction changes, limited airflow and long idle periods can be demanding.

Interrupted regeneration

Repeated short cycles may increase soot loading and, on some engines, contribute to fuel dilution.

Cold towing

The engine may start in extreme cold and then face high load before the complete drivetrain is fully warm.

Seasonal storage

Time limits still apply even when annual mileage is low. Follow the manufacturer’s minimum annual service requirement.

Change before winter when: the oil will become due during the cold season, the truck still contains an unsuitable warm-weather grade, the oil-life monitor is low or contamination is suspected.

Do not reset the oil-life monitor merely because you changed viscosity. Reset it only after the oil and filter have actually been replaced according to the vehicle procedure.

Legacy diesels need their own lookup

Best Cold-Weather Oil for Older Diesel Trucks

An older engine may have been designed around 15W-40, but that does not mean 15W-40 is correct in every winter condition. Many legacy engines permit a lower-W multigrade for cold operation.

Check before changing grade

Original owner’s manual viscosity chart
Engine manufacturer’s current service bulletin
API category allowed for the model year
Turbocharger and injection-system requirements
Lowest oil pressure when fully warm
Oil consumption and leakage history

Do not diagnose with viscosity alone

Slow cranking may be electrical.
Low warm pressure may indicate wear.
Blue smoke may indicate oil consumption.
White smoke may indicate cold combustion or fuel problems.
Turbo noise may require mechanical diagnosis.
Fuel dilution can make oil level rise.

Do not use very thick oil to hide low pressure or consumption. A grade that appears to quiet a worn engine when warm may make cold starting and initial oil delivery worse.

Carry the exact winter oil

How to Top Up Diesel Oil in Cold Weather

Park level and use the correct checking procedure. Follow the manual’s engine-temperature and waiting-time instructions.

Confirm the oil is actually low. A smeared dipstick or cold/hot comparison can create a false reading.

Add the same viscosity and approvals when available. Matching the exact product simplifies the oil mixture and service record.

Add small measured amounts. Allow the oil to drain into the sump before checking again.

Stop at or just below the full mark. Do not overfill for “extra winter protection.”

Investigate repeated loss or a rising level. Oil loss can indicate leakage or burning; a rising diesel oil level can indicate fuel dilution.

Emergency brand mixing: When the engine is genuinely low, an oil with the correct viscosity and required approvals is generally more important than matching the brand. Add only what is necessary and document what was used.

Cold-start troubleshooting

Warning Signs After a Cold Diesel Start

Symptom	Possible concern	What to do
Red oil-pressure light stays on	Insufficient pressure, low oil, wrong viscosity, blocked pickup or mechanical failure	Shut the engine off immediately. Do not continue idling to “warm the oil.”
Persistent knocking or heavy ticking	Delayed lubrication, incorrect oil, injector noise or mechanical damage	Shut down when severe or persistent and arrange diagnosis.
Oil-filter or seal leak	Cold pressure, damaged seal, wrong filter or installation fault	Shut down, repair the leak and restore the correct oil level before restarting.
Cranks slowly despite charged batteries	Oil too viscous, starter/cable fault or internal mechanical resistance	Verify oil grade and perform voltage-drop and starter testing.
Oil level rises	Fuel dilution, coolant contamination, service overfill or incorrect reading	Recheck correctly and diagnose before heavy operation.
Heavy white smoke that does not clear	Cold combustion, injector, compression, glow-system or coolant problem	Do not assume oil is the only cause; arrange diesel diagnosis.

Do not continue operating through an oil-pressure warning. A winter-grade oil cannot protect the engine when the lubrication system is not creating pressure.

Prepare before the first freeze

Diesel Truck Winter Oil and Starting Checklist

Engine oil and filtration

Confirm winter viscosity for the lowest expected temperature.
Verify API, ACEA and OEM approvals.
Change oil and filter if due during winter.
Check for filter, pan, drain-plug and turbo oil leaks.
Carry one sealed bottle of the exact approved oil.
Record oil level before the cold season.

Cold-start systems

Test batteries and charging output.
Inspect battery cables and grounds.
Check glow-plug or intake-heater faults.
Test the block heater and cord.
Verify coolant freeze protection.
Use winterized diesel and maintained fuel filters.

Truck and route preparation

Check tires, including cold inflation pressure and winter suitability.
Confirm DEF level and understand that DEF can freeze while the vehicle’s heater manages normal operation.
Carry suitable recovery, warmth and communication equipment.
Know where the truck can be plugged in overnight.
Plan for colder temperatures at elevation and remote stops.
Avoid beginning a long winter trip with unresolved warning lights.

Ask for the exact product—not “winter oil”

Useful Parts-Counter or Workshop Script

Say this before buying

“My truck is a [year, make and model] with the [engine size and code]. The lowest temperature expected before the next oil change is approximately [temperature]. Please confirm the approved SAE viscosity, API or ACEA category, manufacturer specification, oil capacity with filter and severe-service interval for my VIN.”

Ask the shop to show the approval on the bottle or data sheet. “Suitable for diesel trucks” is not a complete answer.

Cold-climate diesel oil questions

Frequently Asked Questions

What is the best oil for a diesel truck in cold weather?

The best oil is the lowest winter-grade viscosity specifically approved for the truck’s engine and expected temperature. An approved synthetic 5W-40 is common for some cold-climate diesel pickups, while selected engines permit 0W-40 in extreme cold.

Is 5W-40 better than 15W-40 in winter?

When both are approved for the engine, 5W-40 generally provides better low-temperature cranking and pumpability than 15W-40. However, use the manufacturer’s temperature chart rather than changing grades automatically.

Should I use 0W-40 in extreme cold?

Only when the engine manufacturer lists an approved 0W-40. Current Ford 6.7L guidance uses an approved 0W-40 as an alternative at approximately −22°F / −30°C or below, but that recommendation does not apply universally.

Can I use 10W-30 in a diesel truck during winter?

Yes, when the owner’s manual approves it for the expected temperature. Some modern diesels specify 10W-30 for moderate conditions and a synthetic 5W-40 or 0W-40 below a stated temperature.

Is API CK-4 suitable for cold weather?

CK-4 is a diesel performance category rather than one viscosity. CK-4 products are available in several grades, including winter-friendly options. The product must also match the required SAE grade and manufacturer approval.

Can I use API FA-4 instead of CK-4 in winter?

Only if the engine manufacturer explicitly approves FA-4. API states that FA-4 oils are not interchangeable or backward compatible with CK-4 and several earlier diesel categories.

Does synthetic diesel oil help cold starting?

An approved synthetic 0W or 5W oil can provide strong cold cranking and pumpability. Synthetic construction alone is not enough; the viscosity and complete engine specification must match.

When should I use a diesel block heater?

Use it at the temperature and for the duration specified by the truck manufacturer. Several Ford diesel manuals recommend a block heater at approximately −10°F / −23°C or below, but the threshold varies by vehicle.

How long should a diesel idle after a very cold start?

Follow the owner’s manual. Some manufacturers recommend several minutes in extreme cold or use an automatic warm-up strategy. Avoid immediate high load, but do not assume prolonged idling is always best.

Why does my diesel crank slowly even with winter oil?

Possible causes include weak batteries, corroded cables, a worn starter, poor grounds, an unsuitable oil grade or mechanical resistance. Winter oil helps, but the complete starting and fuel systems must also be in good condition.

Official-source record

Official Sources Used for This Cold-Climate Diesel Oil Guide

Oil categories, viscosity charts and cold-weather procedures can change by model year and market. Recheck the exact owner’s manual before buying oil or operating in extreme cold.

Last official-source review: July 30, 2026.

Independent information notice: EngineOiiL-Capacity.com is an independent informational website and is not a vehicle manufacturer, engine manufacturer, lubricant company or repair facility. This guide provides general education and cannot replace the owner’s manual for a specific VIN. Verify the viscosity, API or ACEA category, manufacturer approval, capacity and temperature range before servicing. Shut the engine off immediately for a persistent oil-pressure warning, major oil leak, severe mechanical noise or uncontrolled engine operation.