Diesel engines are powerful and can be resilient in difficult driving conditions. However, your diesel engine is not immune to occasional breakdowns. For instance, these engines will likely experience reduced power output, limiting the overall speed.
A loss of engine power while driving can be a terrifying experience. When this happens, it could mean your diesel engine has gone into derate mode. This issue will trigger a warning notification on the dash, among other symptoms. So, what should you do if your engine slips into derate mode?
This guide explains how to clear engine derate to help your vehicle regain its actual power and speed. But first, let’s see what engine derate means and the symptoms you might encounter.
Question: What is Engine Derate?
Your vehicle’s onboard computer puts the engine into derate mode by limiting its power output. The ECM does this after detecting major faults that can affect the engine’s integrity. Simply put, derate in diesel engines is what “limp mode” means in gasoline engines.
The ECM in your vehicle will constantly monitor the diesel engine with the help of various sensors. It looks into the engine’s fluid levels, temperature, fluid pressure, speed, and more to ensure everything works optimally.
If the ECM detects an issue(s) that could potentially damage the engine or other parts, it initiates a derate mode. The ECM adjusts the engine, ensuring it operates at limited power. It restricts fuel delivery, throttle response, and overall power output so the engine can function safely.
When the engine enters derate mode, it produces moderate power and reduces speed up to 5 mph. This power and speed level allows the operator to drive slowly to the nearest auto repair shop for help. You can also pull over at a safe place and diagnose the problem or seek professional assistance.
Failure to address this issue on time can force your engine to go into a premature shutdown. Note that some manufacturers can disqualify the engine’s warranty in case of a derate-induced shutdown.
Symptoms of Engine Derate
You may observe the following symptoms if your diesel engine slips into derate mode:
- Warning lights appearing on the dash
- Reduced acceleration
- Limited top speed
- Abnormal exhaust emissions
- Unusual engine noises
Answer: How to Clear Engine Derate
To clear engine derate from your vehicle, you must identify the underlying problem and fix it. Remember that the ECM will record DTCs (diagnostic trouble codes) related to the engine derate.
You can use your OBD-II scanner to read these codes. Use the OEM-specific tool if your conventional OBD-II scanner won’t read the engine derate code.
After reading the codes, you will understand the root cause of the engine going into derate mode and fix it. Below are the steps to follow when clearing engine derate:
- Fix the underlying problems. You must diagnose this issue to understand its cause and fix it.
- Delete the engine derate DTC. After resolving all issues, use a scan tool to delete the engine derate code, among other related DTCs.
- Reset the rev counter (tachometer) to zero. Resetting the rev counter will allow your engine to restart at full throttle.
- Reboot the engine (turn the ignition On and Off) three times. Rebooting will put the engine at its normal operation.
- Start the engine and drive for a while. If you’ve fixed the root causes and cleared the engine derate codes, you can drive without experiencing the symptoms mentioned earlier.
Note: What matters most when clearing engine derate is resolving the underlying issue. So, it’s essential to know the common causes of engine derate and how to address them.
Answer: Potential Causes of Engine Derate
The most common causes of engine derate are:
- Blocked air filter
- Restricted exhaust system
- Low oil pressure
- High coolant temperature
- Low fuel pressure
Blocked Air Filter
The leading cause of engine derate is a blocked or clogged air filter. This filter allows clean air to flow into your diesel engine. The filter must be dirt-free to allow sufficient airflow for creating internal combustion.
Unfortunately, this filter collects dirt and debris over time and becomes clogged/blocked. A blocked air filter will discontinue airflow into the engine, resulting in an improper air/fuel mixture ratio. As a result, the engine will run rich and cause incomplete combustion.
With time, the engine will start to overheat and lose power. Continued driving can cause damage to various engine parts. To prevent that, the ECM will initiate the derate mode to protect your engine from damage.
You can address this issue by replacing a dirty or blocked air filter.
Restricted Exhaust System
The exhaust system comprises several components that allow a seamless flow of exhaust gasses. The flow of gasses prevents pressure buildup that can create high exhaust temperatures.
However, issues such as a clogged DPF (diesel particulate filter), low DEF levels, or collapsed pipes will prevent exhaust gasses from exiting. As a result, there’ll be excessive exhaust backpressure and increased temperatures.
Exhaust backpressure can damage the DPF, cylinder heads, and turbochargers. To avoid unnecessary damage, the ECM will derate your engine.
Let’s understand how to clear engine derate by fixing exhaust system problems. If you have a clogged DPF, stop the engine to perform an active regeneration and clean the DPF. Alternatively, remove the filter and clean it with compressed air or a DPF cleaner. You can also clean the exhaust system with a wire brush.
Check the DEF (diesel exhaust fluid) level and top off if necessary. Lastly, inspect the exhaust pipes and allow a professional to repair or replace them if they look collapsed or crimped.
Low Oil Pressure
Low engine oil pressure is another possible cause of engine derate. Usually, the ideal oil pressure should range between 25 and 65 PSI. Maintaining proper oil pressure will ensure sufficient lubrication of engine components.
However, a low oil level, dirty/contaminated oil, or a clogged oil filter can reduce oil pressure. If the pressure goes below 5-7 PSI, the engine will go into derate mode.
To resolve this issue, check the oil level and top off if necessary. You can also change the oil if it looks contaminated or discolored.
Then, replace a damaged or clogged oil filter. If the oil pressure issue persists, seek professional help.
High Coolant Temperature
The engine coolant protects your diesel engine from overheating. If its temperature is too high, your engine will overheat and damage various parts. As a result, the ECM will initiate the engine derate to prevent damage.
Excessive engine coolant temperature can stem from a low coolant level, old or dirty coolant, a faulty thermostat, and a clogged radiator.
When resolving this issue, check the coolant level and condition. Refill the coolant reservoir if the level is low. You’ll need to change dirty or old coolant. Lastly, clean the radiator and replace a failing thermostat.
Low Fuel Pressure
The engine requires adequate fuel flow to ensure complete combustion. If the fuel pressure is low, the engine will run lean, resulting in performance issues. The possible culprits of low fuel pressure include a clogged fuel filter, clogged injectors, and a failing fuel pump.
Low fuel pressure can cause severe damage to various engine parts. To prevent damage, the ECM initiates engine derate to reduce power output.
If you suspect the fuel system is to blame, check the filter and replace it if it’s dirty/clogged. Then, clean the injectors and pour a fuel system cleaner into the tank. Run the engine to allow the cleaner to do its work. If you have a failing pump, take your vehicle to a mechanic for part repair or replacement.
Final Thoughts
Understanding how to clear engine derate can help prevent damage to various engine and exhaust system parts. But to get this job done, you must identify and resolve the root cause of engine derate.
After fixing the underlying problem, use your scan tool to clear any DTCs stored in the ECM. Lastly, test-drive your vehicle to see if you’ve successfully cleared engine derate.