Diesel Engine Derate: 4 Common Causes

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Engine derate is an automatic safety precaution that temporarily reduces power output to protect and prolong the integrity of a diesel engine.

If the ECU detects major faults in a vehicle’s emissions system, it can downregulate the amount of fuel injected into the engine. This drops the vehicle’s speed, and forces the driver to exit the roadway to seek mechanical repairs.

If a diesel engine slips into derate, the vehicle will suddenly lose power and slow to just a few mph. The driver should pull over to check their oil and coolant levels. If the levels or normal, head to the nearest mechanic to have their emissions system inspected. As long as the vehicle remains in derate, it will not accelerate past 5mph.

Suddenly losing power output while driving is always a terrifying experience but by taking time to learn more about engine derate, you can assess the situation and quickly come up with solutions.

In this guide, we’ll explain the types of engine derate and discuss what causes the safety precaution. Then we’ll guide you through how to fix the problem.

What is Engine Derate?

The derate process begins when the ECU receives an error code from the vehicle’s emissions system. A series of sensors located throughout the system record and collect performance data from its six component parts. That information is relayed to the ECU to maintain a safe and balanced exhaust stream.

warning light from ECU error code

These six parts include the:

  • DEF Fluid – Diesel exhaust fluid is a mixture of water and urea that chemically reacts with nitrogen oxides, helping to reduce the amount of air pollution released from a diesel engine.
  • DEF Pump Assembly – The heart of modern diesel emission systems. It is responsible for pumping diesel exhaust fluid to convert toxic NOx gas into nitrogen and water vapor.
  • NOx Sensors – One is located before and one after the SCR catalyst. These sensors measure the amount of NOx in the exhaust stream and report the data to the ECU. When NOx levels rise, these sensors request more urea to offset the amount of toxic gas.
  • Dosing Valve – The component responsible for injecting DEF into the exhaust stream.
  • DEF Control Module – This module regulates DEF functioning and operations.
  • Engine Control Unit – The DEF control module relays information directly to the ECU and, together, the two control systems regulate the entire emissions stream system.

If the ECU receives a faulty code from the DEF control module, it will automatically cut the engine’s power output, forcing the vehicle to a crawl until the problem has been resolved. This helps to ensure that the engine runs at peak performance without polluting the surrounding environment or damaging the engine.

How to Tell If Your Engine is in Derate

The most obvious way to tell if your vehicle’s engine has slipped into derate is if it suddenly loses power. Of course, you should ideally try to prevent derate by regularly inspecting and maintaining your engine’s emissions system. If you notice any of the following signs, your engine may need some upkeep to prevent a sudden malfunction.

A Derate Warning

Some vehicles will issue a warning before they automatically go into derate mode. If your vehicle is equipped with this feature, a light on the dash will illuminate, often accompanied by a beeping noise. The message will either say “check engine,” “service engine soon,” or “engine derate in 3 hours”.

engine derate message

If you see this message, immediately pull over to the side of the road and turn off the engine. If you attempt to drive any further, it could damage your engine and throw your vehicle in derate while you’re driving in busy traffic.

Decreased Performance

If your engine has recently taken time to start up in the morning or is losing power while driving, it may not be getting enough DEF fluid. This means that your dosing valve may be malfunctioning or that there’s a hole in your DEF tank. If this is the case, you’ll need to take your vehicle to the nearest service station to have it refilled.

If you notice a sudden drop in performance, your NOx sensors may also need replacing. These sensors are located before and after the SCR catalyst and, if they’re malfunctioning, they will mismeasure the amount of NOx in your engine’s exhaust stream. When this happens, the ECU will not receive necessary information and derate the engine to prevent further damage.

Most Common Causes of Engine Derate

There are several reasons why a vehicle’s engine may slip into derate mode but, more often than not, it’s due to a problem with the SCR/DEF system. If any of the following occurs, your engine may automatically reduce its power output.

derate instruction chart

Low DEF Levels

If your vehicle’s DEF tank runs dry, the ECU will automatically go into derate mode to prevent your engine from running without the necessary fluid. This is because, without DEF, your engine will produce harmful NOx emissions that can damage the environment and pollute the air.

DEF Dosing Valve Failure

The dosing valve is responsible for injecting DEF into the exhaust stream and, if it malfunctions, your engine will be forced into derate. This is because the ECU will not receive the correct information about the amount of DEF in the system. As a result, will not be able to properly regulate the engine’s emissions.

DEF Pump Assembly Failure

The DEF pump assembly is responsible for pumping DEF fluid to the SCR catalyst. If this component fails, it can cause a build-up of NOx emissions that can pollute the air.

SCR Catalyst Failure

The SCR catalyst is responsible for converting NOx emissions into nitrogen and water vapor. If this component fails, it can cause a build-up of NOx emissions that can pollute the air.

In most cases, you’ll receive a check engine code with specific information about what caused your SCR/DEF system to malfunction. These codes vary slightly but include:

  • SPN 5246 FMI 0
  • DEF SCR Inducement Faults 
  • P103C
  • P207F
  • P208E
  • P103B

Any service center should be able to read these codes and provide further information about the necessary repairs. 

Can You Temporarily Override 5 MPH Engine Derate?

If you’re driving on the highway and your engine suddenly goes into derate, it can be a very scary experience. Most engines will automatically reduce power output to just 5 mph but, in some cases, you may be able to override this safety precaution. If possible, pull over to the side of the road, stop your engine, and disconnect the speed sensor from the transmission.

This will limit your engines RPMs to around 1400 but you should be able to drive at reasonable speeds until you reach a mechanic. This is not a long-term solution, though, and should only be attempted in absolute emergencies. Driving without a speed sensor while your engine’s emissions system is damaged can cause untold damage and create massive amounts of pollution.

How to Fix an Engine in Derate?

If your engine has slipped into derate, the best thing you can do is turn off the engine and call a tow truck. Once you’re at a safe location, you can assess the situation and decide how to proceed. In most cases, you can fix the problem by topping up your DEF fluid, replacing your dosing valve, or repairing your SCR catalyst.

If you have experience working on a diesel engine’s emissions system, you may be able to fix these issues yourself. However, if you’re unsure of the problem or lack the experience to make the repairs, the best course of action is to take your vehicle to a reputable service station.

Improperly adjusting your vehicle’s emissions system can cause further damage to the engine. Also, it can cause air pollution, and go against federal emissions regulations. This is especially true if there’s a problem with the DEF control module, which should be tuned to your vehicle before you attempt to run the engine.

The Bottom Line

Engine derate is a pre-programmed safety measure designed to drop power output if the engine’s emissions system malfunctions. This helps to protect the engine from damage and prevent air pollution. So, if your engine suddenly loses power while you’re driving, it’s likely slipped into derate. To fix the problem, head to your nearest service station and have the emissions system inspected.

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