What Is Diesel Engine Derating and Why Does It Happen?
Updated: Oct 6
If you've ever had a diesel truck, semi, or motorhome suddenly lose power and heard someone say, “The engine is derated,” you may wonder what that actually means.
A derate occurs when the vehicle's control system intentionally limits engine power, torque, RPM, vehicle speed, or some combination of them because it has detected a condition that requires attention.
In other words, the engine may be physically capable of making more power, but the computer is intentionally not allowing it to.
Depending on the problem and the vehicle, a derate can range from a slight reduction in performance to a severe speed or power limitation.
The important thing to understand is:
A derate is a response to a problem. It is not the diagnosis.
Why Would a Diesel Intentionally Reduce Power?
Modern diesel engines constantly monitor operating conditions through sensors and control modules.
If the system detects certain problems, reducing engine output can help:
Protect the engine from damage
Protect the turbocharger or aftertreatment system
Prevent excessive temperatures or pressures
Respond to emissions-system faults
Encourage correction of certain DEF/SCR problems
Prevent continued operation under potentially damaging conditions
On SCR-equipped engines, certain DEF or emissions-system conditions can also trigger progressively stronger power or speed limitations.
What Does a Derate Feel Like?
The symptoms depend on how severe the derate is.
A driver may notice:
Reduced acceleration
Loss of pulling power
Limited engine RPM
Limited road speed
Poor performance climbing grades
Reduced throttle response
Check-engine or warning lights
DEF or aftertreatment warnings
Messages indicating reduced engine power
In severe cases, a very low maximum vehicle speed
Sometimes the truck still drives relatively normally but doesn't make full power.
Other times, the reduction can be dramatic.
Derate vs. Limp Mode
The terms are sometimes used interchangeably, but they are not always exactly the same.
A derate generally means the vehicle is intentionally reducing power or speed because of a detected problem, while limp mode is a broader fail-safe strategy that may also affect things like transmission shifting.
The exact behavior depends on the manufacturer and the fault involved.
Not Every Loss of Power Is a Derate
This is an important distinction.
A truck that feels weak isn't automatically derated.
A diesel can lose power because of an actual mechanical problem such as:
Low fuel pressure
Restricted fuel filter
Turbocharger problem
Charge-air leak
Restricted air filter
Exhaust restriction
Injector problem
Transmission problem
With a true electronic derate, the control system is intentionally commanding reduced performance because of a detected condition.
Proper diagnostics can help determine whether the vehicle is actually being electronically limited or simply cannot produce normal power because something isn't working correctly.
Aftertreatment Problems Are a Common Cause
Modern diesel aftertreatment systems are one area where drivers commonly encounter derates.
These systems can include:
DOC — Diesel Oxidation Catalyst
DPF — Diesel Particulate Filter
DEF dosing system
SCR — Selective Catalytic Reduction
NOx sensors
Exhaust temperature sensors
DPF pressure sensors
If the control system determines that the aftertreatment system isn't operating correctly, warning stages may occur before power, or vehicle speed is reduced.
Depending on the engine and calibration, the system may provide several warnings before a severe derate occurs.
Low DEF Can Cause a Derate
DEF isn't optional on an engine designed to use it.
The SCR system relies on Diesel Exhaust Fluid to help control NOx emissions.
As DEF level becomes low, the vehicle normally begins warning the driver.
If those warnings are ignored and DEF becomes critically low, some systems can progressively limit engine performance or vehicle speed.
Simply adding DEF may correct a low-fluid condition, but a DEF-related warning isn't always caused by the tank simply being low.
DEF Quality Problems
The system also cares about what is inside the DEF tank.
Incorrect, diluted, contaminated, or improperly concentrated DEF can prevent the SCR system from operating correctly.
Modern diesel systems can monitor DEF quality and detect situations where the SCR system is not reducing emissions as expected.
So, filling the DEF tank doesn't automatically solve every DEF-related fault.
The problem could involve:
DEF quality
DEF pump
DEF injector
DEF heater
Wiring
Sensors
SCR performance
DPF Problems Can Contribute to Reduced Power
The DPF captures particulate matter and soot.
As soot accumulates, the system uses regeneration to burn much of that material away.
If the DPF becomes excessively loaded or the truck repeatedly fails to complete regeneration, exhaust restriction can increase and faults can develop.
Potential causes of DPF-related problems can include:
Failed or interrupted regens
Excessive soot production
DPF pressure sensor problems
Exhaust temperature sensor problems
Engine problems
Turbocharger issues
Injector problems
EGR problems
Excessive ash accumulation
This is why a DPF fault does not automatically mean the DPF needs to be replaced.
The reason the DPF is having a problem still needs to be determined.
Sensor Problems Can Cause Derates
Modern diesel engines depend heavily on accurate sensor information.
Depending on the system, the control modules may monitor:
Coolant temperature
Oil pressure
Fuel pressure
Boost pressure
Intake temperature
Exhaust temperature
DPF pressure
NOx levels
DEF quality and level
Engine speed and position
If the control system receives information indicating an unsafe or incorrect operating condition, it may limit engine performance.
But there is another important possibility:
Sometimes the sensor or circuit reporting the problem is itself faulty.
For example, an incorrect pressure reading could be caused by:
The actual pressure being wrong
A failed sensor
Damaged wiring
A connector problem
Poor power or ground
A plugged sensing tube or passage
That's why reading a fault code is only the beginning of the diagnostic process.
Overheating Can Lead to Reduced Power
High engine temperature is another condition that can cause protective strategies.
If coolant temperature becomes excessive, reducing engine output can reduce the amount of heat the engine is producing.
The underlying overheating problem might involve:
Low coolant
Coolant leak
Restricted radiator
Fan clutch problem
Water pump issue
Thermostat problem
Airflow restriction
Cooling fan control problem
Dirty radiator or charge-air cooler
This becomes especially important when towing, climbing grades, or operating in Arizona summer temperatures.
A truck that only overheats under heavy load may have a cooling system that can handle normal driving but cannot keep up when engine demand increases.
Fuel-System Problems
Fuel pressure is another critical operating parameter.
Modern diesel fuel systems operate within specific pressure ranges, and serious fuel-pressure problems can result in poor performance, fault codes, or protective engine strategies.
Possible causes include:
Restricted fuel filters
Weak supply pump
High-pressure pump problems
Fuel-pressure control problems
Injector issues
Air entering the fuel system
Fuel supply restrictions
Again, the code may identify the system experiencing the problem, not necessarily the exact component that failed.
Turbocharger and Boost Problems
A diesel engine relies heavily on boost pressure to make power.
If actual boost doesn't match what the engine computer expects, the system may detect an under boost, over boost, or turbo-control problem.
Possible causes include:
Turbocharger failure
VGT actuator problem
Sticking VGT mechanism
Charge-air cooler leak
Loose or damaged boost boot
Intake restriction
Exhaust leak
Sensor problem
If the truck has a low-boost fault, replacing the turbo without checking the rest of the system can become an expensive mistake.
Why Does the Derate Sometimes Get Worse?
Some systems use progressive warnings and limitations.
The initial fault may trigger a warning light or message.
If the condition continues, isn't corrected, or reaches another threshold, the system may impose a stronger limitation.
That's why a driver may initially notice only a warning and later experience significantly reduced power or vehicle speed.
The exact sequence depends on the engine manufacturer, application, model year, fault, and calibration.
Can You Just Clear the Code?
Sometimes drivers want to clear the codes and see if the derate disappears.
That doesn't repair the problem.
Even if a warning temporarily disappears, the control system may detect the same condition again.
If the original cause is still present:
The code may return
The warning light may return
The derate may return
The problem may become worse
Fault codes are useful diagnostic information.
Clearing them without understanding why they were set can also remove information that helps diagnose the vehicle.
Can You Just Perform a Forced Regen?
Not every derate is solved by performing a parked or forced regeneration.
If the underlying problem is:
A failed temperature sensor
Bad DPF pressure reading
Fuel-system problem
Turbocharger problem
DEF fault
NOx sensor issue
Exhaust leak
Excessive DPF ash
Wiring problem
forcing another regen may do nothing to correct the actual cause.
Before performing repeated forced regens, the reason the truck isn't regenerating normally should be understood.
Why Proper Diagnosis Matters
When a diesel arrives with a derate, the first question shouldn't be:
“Which part do we replace?”
The better question is:
“What condition caused the control system to reduce power?”
Diagnosis may involve:
Reading active and inactive fault codes
Reviewing fault history
Monitoring live data
Checking engine temperatures
Checking fuel pressure
Comparing commanded and actual boost
Inspecting the turbo and charge-air system
Checking DPF restriction
Reviewing regeneration history
Testing exhaust temperature sensors
Testing NOx sensors
Checking DEF quality and dosing
Inspecting wiring and connectors
Road testing the vehicle
The testing depends on the fault and what the vehicle is actually doing.
Tell the Shop What Happened Before the Derate
Information from the driver can be extremely useful.
Try to remember:
What warning appeared first?
Did the truck gradually lose power or suddenly lose power?
Was the vehicle towing or heavily loaded?
Were you climbing a grade?
Did engine temperature increase?
Did the truck recently request a regen?
Was DEF recently added?
Did the problem occur after fueling?
Has the truck been performing frequent regens?
Were there any unusual noises, smoke, or smells?
How long had the warning been present before the derate?
Those details can help point the diagnosis in the right direction.
Don't Ignore the Warning Stages
One of the biggest mistakes a driver can make is continuing to operate the vehicle while warning lights and messages continue getting worse.
A warning doesn't always mean you need to immediately shut the truck down, but it does mean you should understand what the warning is telling you.
Follow the manufacturer's instructions for that specific warning.
If a warning progresses, power begins dropping, or a stop-engine warning appears, the condition should be taken seriously.
Addressing the problem earlier may prevent additional downtime or damage.
The Bottom Line
A diesel engine derate means the control system is intentionally reducing performance because it has detected a condition that requires attention.
The cause may involve:
DPF or aftertreatment problems
DEF level or quality
SCR faults
Sensors
Fuel pressure
Turbocharger or boost
Engine temperature
Electrical problems
Other engine-protection conditions
But the derate itself doesn't tell you which part has failed.
The derate is the response. The job of diagnosis is finding the cause.
Don't automatically replace the component mentioned in a fault code, repeatedly force regens, or simply clear codes and hope the problem goes away.
Read the faults. Test the system. Find the cause. Then make the repair.
At Deer Valley Diesel, we diagnose derate, aftertreatment, fuel, turbocharger, electrical, and drivability problems on diesel trucks, semis, and diesel RVs throughout Phoenix, Deer Valley, and North Phoenix.
