What Causes a Diesel Truck to Go Into Derate?

Give California Mobile Fleet the opportunity to service your needs whether it's an Emergency or just routine scheduled maintenance. 

We're here to help 24 hours a day!

What Are Common Causes of Diesel Truck Derate?

There is not one single cause of diesel derate. Modern heavy-duty trucks monitor many engine, emissions, and electronic systems. A fault in one of those areas may lead to reduced performance depending on the vehicle and severity of the condition.

Possible areas involved in a derate condition can include:

• Aftertreatment system problems
• DPF-related problems
• DEF/SCR system faults
• Sensor or electrical faults
• Engine-related faults
• Exhaust-system conditions
• Wiring or connection problems
• Other monitored system faults

Because many different conditions can produce reduced power, diagnosing the truck based only on the fact that it is “in derate” is not enough.



The cause needs to be identified.

Can Aftertreatment Problems Cause a Diesel Truck to Derate?

Yes. Aftertreatment-related faults are one category that can contribute to derate conditions on modern diesel trucks.

The aftertreatment system works with the truck's electronic controls and emissions-related components. When the system detects certain problems, warning lights or dash messages may appear, and depending on the fault and vehicle, operating performance may eventually be affected.

Potential areas requiring investigation may include:

• Diesel particulate filter (DPF)
• Diesel exhaust fluid (DEF) system
• Selective catalytic reduction (SCR) system
• Sensors used by the aftertreatment system
• Wiring and electrical connections
• Other aftertreatment components or operating conditions

An aftertreatment warning does not automatically identify which component has failed.

Proper diesel diagnostics are important for determining what the truck is actually reporting and why.

Can a DPF Problem Cause a Diesel Truck to Derate?

A DPF-related condition can be associated with warning lights, regeneration concerns, fault codes, and in some situations reduced engine performance.

The diesel particulate filter is part of the truck's aftertreatment system. Over time, soot accumulates in the filter and must be managed through the system's normal regeneration process.

If the system cannot complete the process as expected, or another fault interferes with proper aftertreatment operation, the truck may begin displaying warnings.

The important point is that a DPF warning or related fault does not automatically mean the filter itself should be replaced.



The truck may need to be diagnosed to determine whether the issue involves soot loading, a sensor, another aftertreatment component, an operating condition, or a different fault affecting the system.

Can DEF Problems Cause a Diesel Truck to Lose Power?

DEF and SCR system problems can contribute to emissions-related warnings and operating restrictions depending on the truck and the condition detected.

A problem may involve more than simply the amount of DEF in the tank.

The system contains multiple components and electronic controls, so a DEF-related warning may require investigation of the system rather than immediately assuming that adding fluid will solve the problem.

If the truck displays DEF, SCR, emissions, or related warning messages, pay attention to exactly what the dash is reporting.



Those messages, along with fault codes and diagnostic information, can help a technician determine where to begin testing.

Can a Bad Sensor Put a Diesel Truck Into Derate?

Sensor and electrical problems can contribute to conditions that result in reduced engine performance.

Modern diesel trucks rely on sensor information to monitor engine and aftertreatment operation. If a signal is missing, incorrect, out of expected range, or affected by wiring or connection problems, the control system may detect a fault.

That does not mean every sensor-related fault code proves the sensor itself is bad.

The actual cause may involve:

• The sensor
• Wiring
• Connectors
• Power or ground
• Another component affecting the sensor reading
• A mechanical condition creating an abnormal reading

This is why replacing the component named in a fault code without testing can become expensive. The code is information. Diagnosis determines what that information actually means.

What Warning Signs Can Appear Before or During a Derate?

A truck may provide warning signs before or while performance becomes limited.

Depending on the vehicle and fault, the driver may notice:

• Check engine light
• Stop engine warning
• DPF warning light
• DEF or SCR-related warning
• Emissions-system messages
• Reduced engine power
• Poor acceleration
• Limited speed
• Difficulty pulling a load
• Changes in engine performance

Do not assume that two warning lights appearing together necessarily have two unrelated causes.



Modern truck systems interact with each other, and diagnostic testing can help determine which faults are active, which are related, and which information matters to the current problem.

Why Fault Codes Should Be Diagnosed Instead of Guessed At

Fault codes are valuable because they tell a technician what the truck's control system has detected. They are not always a direct instruction to replace the component mentioned in the code description.

A proper diagnostic process may involve:

• Retrieving active and stored fault codes
• Reviewing relevant live data
• Checking sensor readings
• Inspecting wiring and connectors
• Performing electrical tests
• Inspecting related mechanical components
• Evaluating aftertreatment conditions
• Comparing diagnostic information with the truck's actual symptoms

For example, a sensor-related fault may ultimately be caused by the sensor itself, wiring, a connection problem, or another system condition affecting the reading.



Testing helps separate the symptom from the cause.

That can reduce unnecessary parts replacement and help identify the repair the truck actually needs.

What Should You Do When a Diesel Truck Goes Into Derate?

Start by paying attention to what the truck is telling you.

Note the warning lights, dash messages, changes in power, and anything that happened immediately before the derate began.

Useful information includes:

• Which warning lights are illuminated
• Exact dash messages, if available
• Whether power loss happened suddenly or gradually
• Whether the truck recently attempted a regeneration
• Any recent DEF or aftertreatment warnings
• Whether the truck is still operating normally enough to move safely
• Any recent repairs or recurring fault codes

Avoid assuming that clearing a code means the underlying problem has been repaired. If the condition that caused the fault is still present, further diagnosis may be needed.

When a derate leaves the truck unable to continue safely or normally, emergency roadside assistance may allow the problem to be assessed where the truck is located.

Can a Derated Diesel Truck Be Diagnosed Roadside?

In many situations, initial diagnosis can be performed where the truck is located.

A mobile technician may be able to connect diagnostic equipment, retrieve fault information, inspect related components, perform electrical testing, and evaluate the symptoms without automatically moving the truck to a shop first.

That can be especially useful when the truck is at a roadside location, fleet yard, truck stop, distribution facility, or job site.

Depending on what the diagnosis finds, mobile diesel repair may be able to address the problem on location.

Other failures may require additional equipment, parts, procedures, or facility-based repair. The purpose of roadside diagnosis is not to promise that every derate can be fixed on-site. It is to determine what is wrong and what the appropriate next step should be.



This distinction follows CMF's mobile-first positioning while avoiding the claim that every problem can be repaired roadside.

Does a Derated Diesel Truck Always Need to Be Towed?

No. A derate does not automatically mean the truck needs a tow. Some problems may be diagnosed and, depending on the failure, repaired where the truck is located.

Other conditions may make continued operation inappropriate or require repairs that cannot reasonably be completed roadside.

Towing may be the better option when:

• The truck cannot be operated safely
• The required repair cannot be completed on location
• Major mechanical problems are suspected
• Specialized facility equipment is required
• Roadside conditions make the repair unsafe or impractical

If on-site repair is not appropriate, towing service may be the next step after the problem has been assessed.

The goal is not to avoid towing at all costs. It is to avoid an unnecessary tow when mobile diagnosis or repair is a practical option. That is directly consistent with CMF's stated service philosophy.