How to Clear Engine Derate on Peterbilt: Step-by-Step Guide

As a long-haul trucker, I understand the engine derate meaning all too well. One minute you’re cruising down the interstate, and the next your truck inexplicably starts losing power. Before you know it, you’re crawling along at half speed, wondering if you’ll make your delivery appointment on time. I have … Read more

Photo of author

Written by: Mohammad Sameer

Updated on:

As a long-haul trucker, I understand the engine derate meaning all too well.

One minute you’re cruising down the interstate, and the next your truck inexplicably starts losing power.

Before you know it, you’re crawling along at half speed, wondering if you’ll make your delivery appointment on time.

I have become skilled at resolving engine derate issues on my Peterbilt and fixing them when they occur.

In this post, I will share my practical knowledge to help Peterbilt owners and operators understand everything they need to know.

Whether you are an independent trucker or a fleet manager, you will learn about the causes of engine derates, how to troubleshoot them, and how to prevent these frustrating power limitations from happening in the future.

What Triggers Peterbilt Engine Derates

image of What Triggers Peterbilt Engine Derates
image source:

Essentially, an engine derate definition includes the scenario when the engine control module (ECM) determines something is amiss under the hood and initiates a protective measure to save your engine from damage. Instead of operating at full speed and horsepower, the ECM intentionally limits power output.

Now you might be wondering – why would the computer do this instead of just illuminating a warning light? Well, some truckers may be tempted to ignore a small warning light if they’re on a deadline. But you definitely can’t ignore a significant loss of speed! The engine derate in 3 hours forces you to stop and investigate what’s going on.

In my experience, these are the most common triggers for Peterbilt engine derates:

Excessive Exhaust Temps

Diesel engines produce extreme heat and exhaust temperatures. If sensors detect exhaust levels exceeding normal parameters, it likely means there are issues like a dpf full engine derated, prompting immediate attention.

  • Leaking injectors misfiring
  • Fouled or plugged fuel injectors
  • A clogged or failed turbocharger

In these cases, the ECM derates the engine to prevent thermal damage.

Low Oil or Coolant Pressure

Pressure sensors in your Peterbilt track oil and coolant levels. If they start dropping below expected PSI levels, it can indicate major leaks, blocked passages, coolant leaks, or a failing water pump. Left unchecked, these issues can lead to a peterbilt engine derate or overheat or seize your entire engine – not good when you’re cruising down the freeway!

Clogged Air, Fuel and Oil Filters

You know how they say engines need proper ‘breathing’ to run correctly? Well, it’s true! Over time, air, fuel, and oil filters inevitably get contaminated with dust, soot, and particles. As they become increasingly restricted, your engine isn’t able to inhale the volumes of air and fuel needed to reach optimal combustion, which is why a clean peterbilt air filter is essential.

Failing Sensors

Modern trucks are equipped with an extensive engine codes list of sensors that monitor everything from oil temperature to crankshaft position. If any of these sensors begin to malfunction or relay inaccurate readings to the ECM, it can erroneously activate an engine derate. I’ve observed both temperature and pressure sensors intermittently failing, leading to a variety of unusual problems.

Electrical Problems

Electrical gremlins or glitches within the ECM programming itself can sometimes mistakenly initiate a derate mode in your truck. Resolving this typically necessitates dealer-level software and advanced ecm reset and debugging capabilities to properly diagnose and rectify the issue.

The Frustrating Symptoms of Peterbilt Engine Derates

image of The Frustrating Symptoms of Peterbilt Engine Derates
image source:

How can you tell if your reliable Peterbilt has unintentionally entered engine derate mode, restricting your speed and power? Be on high alert for these symptoms, which may indicate an engine derate code has been triggered:

Significant Loss of Power

You’ll immediately recognize a significant change in acceleration and pulling power when your engine computer imposes a level 1 engine derate. Your truck will become noticeably sluggish as engine RPMs are capped, with hills and inclines particularly underscoring the reduction in horsepower.

Inability to Maintain Speed

Cruising down the highway at a steady pace? The moment a powertrain derate takes effect, you’ll struggle to keep up your speed, even at reduced velocities like 55 or 60 MPH. The ECM is deliberately slowing you down to avert catastrophic failure.

Increased Fuel Consumption

Here’s a frustrating truth – despite engine derates cutting your speed and power, they also diminish fuel efficiency. The resulting lugging, misfiring, and irregular combustion means your engine consumes additional diesel just to chug along the road, a situation that fleet managers, especially those dealing with an emission derate Peterbilt, find distressing due to the surge in fuel expenses.

Illuminated Warning Lights

While the substantial loss of power itself should capture your attention, most Peterbilts will also signal the issue by illuminating a series of warning and fault indicator lights on the dashboard, including the peterbilt check engine light.

  • Check Engine Light
  • Temperature Warning Light
  • Oil Pressure Warning
  • DPF Light

These represent the different sensors and parameters causing your ECM to limit speed.

How to Clear Engine Derate on Peterbilt?

Okay, so you’ve pinpointed the symptoms and realize your Peterbilt has entered self-preservation mode with an engine derate.

Now it’s go time – you need to systematically troubleshoot and execute the necessary repairs to clear the engine derate, allowing your truck to resume normal operation.

Over the years and dealing with my trucks, I’ve honed a reliable 4-step game plan, as detailed in the Peterbilt service manual, for diagnosing and clearing Peterbilt engine derates:

Step 1: Scan and Diagnose All Active Fault Codes

image of Scan and Diagnose All Active Fault Codes
image source:

This first step is crucial – you’ll need a Peterbilt code reader to access and interpret any diagnostic trouble codes (DTCs) stored in your truck’s computer.

These codes are pivotal in pinpointing which engine parameters are out of spec and responsible for the derate.

For Peterbilts, I suggest employing an OEM-level scan tool like Noregon’s JPRO system, which is adept at uncovering proprietary Cummins and PACCAR Peterbilt fault codes that generic code readers can’t detect.

Jot down the Trouble Codes and commence your investigation!

Step 2: Thoroughly Inspect and Repair Mechanical Issues

Here’s where the true repair work begins. Once you’ve diagnosed the fault codes and pinpointed the failing engine components involved with the derate, it’s time to physically inspect and rectify the underlying mechanical issues.

Don’t just reset the check engine light and delete derate counters without tackling the root causes – that approach only offers a temporary fix before the problems reemerge. Instead, be prepared to roll up your sleeves and resolve the real issues like:

  • Replacing worn injectors
  • Cleaning diesel particulate filters
  • Fixing leaky gaskets or seals
  • Swapping out bad sensors or relays
  • Unclogging blocked fuel supply lines

This takes time and getting your hands dirty, but is the only way to permanently clear engine derates.

Band-aid fixes might work in the short term, but they won’t hold up in the long run. Do it right the first time.

Step 3: Reset Derating Counters

Alright, after you’ve scanned codes and fixed all underlying hardware problems, replaced damaged components, and restored your Peterbilt engine to full operational health, it’s essential to reset the engine control module to clear the derate gremlins from your truck’s computer system.

Using your OEM scan tool, access the Engine Control Module (ECM) and look for a sub-menu to reset derating counters.

This ecm reset tells your ECM that any prior issues forcing power limitations have been successfully repaired – essentially giving it a clean slate to resume normal operation.

Step 4: Carefully Retest at Full Power

The moment of truth has arrived. Turn the key, fire up your Peterbilt’s engine, say a little prayer, and begin cautiously testing it under hard acceleration at full rated horsepower.

Gradually accelerate, climb hills, pull heavy loads.

Closely monitor warning lights and check engine codes for the first 100 miles or so to ensure no lingering gremlins remain.

I also recommend gently ‘breaking in’ your engine the first 1,000 miles post-repair to get valves, cylinders, and components properly re-seated.

If you don’t see any new issues arise after a few weeks, congratulations – through proper troubleshooting and mechanical fixes, you successfully cleared that pesky engine derate!

Proactive Maintenance Prevents Problems

Want to minimize engine derates plaguing your Peterbilt in the first place? An ounce of prevention is truly worth a pound of cure when it comes to Peterbilt maintenance.

Stay religious about these proactive upkeep measures for a healthy, derate-free engine:

Change Oil Every 15,000 Miles

Extending intervals beyond factory recommendations just asks for accelerated wear, corrosion, and sludge that clogs filters faster, including your Peterbilt radiator. Spend a little now on preventative maintenance to avoid huge repair bills down the road.

Replace Air & Fuel Filters

Peterbilt fuel filters and air filters are essential for maintaining engine health, capturing particles down to 10 microns. However, after months of heavy highway use, they can become clogged with soot and debris. It’s advisable to replace both the air and peterbilt fuel filters twice a year or every 50,000 miles to ensure optimal performance.

Flush & Refill Coolant System

Peterbilt coolant, crucial for engine temperature regulation, degrades over time due to heat cycling. It’s important to test its strength at least yearly with test strips. Should the concentration fall below 60%, it’s time to drain the old coolant and refill with fresh premixed peterbilt coolant rated to -20 degrees F.

Install Coolant Filter

Installing a peterbilt coolant filter can significantly enhance the cleanliness of your cooling system by acting as an extra preventative measure. These compact canister filters are adept at trapping rust flakes and scale particles, ensuring your engine runs smoother for longer.

Use Quality Engine Oils

Peterbilt engine oil technology has seen significant advancements. Investing in advanced full synthetic oils, which include detergent additives and viscosity stabilizers, is a wise choice. These oils not only flow better in cold conditions but also endure higher temperatures and maintain their integrity, far surpassing the performance of older conventional oils.

Monitor Engine Fault Codes

Make it a routine to periodically check for diesel engine codes, even when your truck appears to be functioning properly. This proactive approach helps identify minor issues before they escalate into major failures that could lead to derates. Consider it a form of preventative maintenance for your vehicle.

Address Problems Early

Should you encounter engine light codes, strange noises, or changes in performance, it’s critical to address these signs immediately. Early repairs are generally more cost-effective, while delaying can lead to more severe problems, potentially resulting in a major breakdown and expensive roadside repairs.

Tune-Ups Keep Things Humming

Even the finest engines, like those in Peterbilt trucks, benefit from regular tune-ups. At a minimum, every two years, have a professional perform a peterbilt tune-up, which should include a thorough inspection of the engine system, compression checks, timing assurance, turbocharger boost testing, and more to preemptively tackle potential issues.

By adhering to proven maintenance principles and avoiding driving your Peterbilt to its limits, you’ll ensure a smooth-running engine and significantly reduce the likelihood of PACCAR powertrain derate events.

The Inside Scoop on Diagnosing Peterbilt Trouble Codes

Now, let’s delve into the details of how to utilize engine fault code definitions to accurately identify the causes of Peterbilt derates.

It’s important to start with the essential step of employing a diagnostic scanner to retrieve the complete engine codes list from the Diagnostic Trouble Codes (DTCs) stored in your truck’s Engine Control Module.

For trucks still under warranty, dealership service centers are equipped with the necessary tools and expertise to decipher Peterbilt fault codes. However, for others, interpreting these codes requires some investigative skills to determine which engine components need attention.

Let’s proceed to examine a selection of common derate-related Peterbilt fault code analysis to aid in troubleshooting.

SPN 4364 / FMI 18 – EGR Valve Position Sensor Fault

image of EGR Valve Position Sensor Fault
image source:
  • SPN = Suspect Parameter Number. Identifies the specific component affected
  • FMI = Failure Mode Indicator. Describes the type of failure

Encountered this code? It suggests that the Peterbilt EGR valve position sensor is relaying inaccurate information to the ECM, which is vital for regulating the timing of exhaust gas recirculation into the intake to control combustion temperatures.

When the ECM detects incorrect data from the position sensor, it may conclude there’s a problem with the Peterbilt emissions system, prompting a derate to safeguard the engine until the sensor issue is resolved.

SPN 5246 / FMI 31 – EGR Cooler Performance Fault

The peterbilt dpf regeneration system is crucial for limiting emissions, and a fault here often points to issues with your diesel particulate filter (DPF) regeneration system. Specifically, this fault may arise from insufficient exhaust cooling, a problem typically associated with the EGR mechanism.

When triggered, investigate your entire EGR subsystem for issues:

  • Sticking EGR valve
  • Leaking or cracked EGR cooler line
  • Overheating EGR cooler assembly
  • Exhaust leaks pre or post EGR entrance

Peterbilt EGR cooling is essential for maintaining engine performance, and any conditions that diminish its efficiency can trigger low performance faults, leading to a derate in power and compromised cooling effect.

SPN 157 / FMI 5 – High Engine Coolant Temperature

A critical alert for any Peterbilt vehicle is when the peterbilt coolant temperature sensor logs readings that soar beyond the normal operational range, flagging potential overheating issues that demand immediate attention.

Peterbilt overheating can start with something as simple as low coolant levels, but significant over-temp warnings are often harbingers of more severe cooling system issues like:

  • Extremely low coolant
  • Damaged water pump impeller
  • Stuck thermostat
  • Cracked radiator end tanks
  • Cooling fan clutch failure

Ignoring peterbilt cooling system issues can quickly escalate to serious damage, such as blown head gaskets, cracked cylinder heads, and even complete engine seizures – all outcomes that must be avoided at all costs.

SPN 100 / FMI 4 – Oil Pressure Critically Low

image of SPN 100 / FMI 4 - Oil Pressure Critically Low
image source:

Encountering a ‘critically low oil pressure’ SPN code is akin to entering a trucker’s worst nightmare. This is a dire warning from your engine, and for Peterbilt vehicles, it precedes the devastating damage that can occur when rod bearings and crankshaft journals begin to fail during operation.

Don’t ignore this code! Immediately investigate causes like:

  • Extremely low oil level
  • Faulty oil pressure sensor
  • Damaged or blocked oil pickup tube
  • Excess bearing clearance from wear
  • Oil pump issues or leaks

Peterbilt engine failure is a costly affair, and low oil pressure is a red flag that requires immediate intervention to prevent bearing seizure, which can lead to catastrophic engine failure and the prospect of an expensive $20,000+ rebuild.

As you can see from just a small sampling, properly diagnosed peterbilt SPN fault codes guide troubleshooting of both minor and major issues. Combining these codes with physical inspections pinpoints root causes behind pesky Peterbilt engine derates plaguing your truck!

Avoiding Dealership Ripoffs on Engine Derates

Considering the experience I’ve gained wrenching on Peterbilts over decades on the road, dealership service centers still try pulling fast ones over on me regarding engine derates! After repairs, I’ve had techs insist that Peterbilt ECM reprogramming is ‘mandatory’ before a derated truck resumes full power operation.

They claim proprietary factory software tuning has adapted to mechanical issues and requires an expensive ‘hard reset’ – conveniently billable hours for them. Don’t fall for this scam perpetuated by some peterbilt dealership service centres!

Unless engine internals like turbo, injectors or compression ratios underwent physical changes needing software matched, merely clear derate counters through standard peterbilt OBD-II protocols using a quality scan tool. That properly alerts the ECM that hardware fixes addressing root causes were completed successfully.

If dealership technicians insist existing peterbilt engine calibrations are now unsuitable without further programming, ask them to explain what specific parameters have shifted requiring software resets. Usually, they are unable to provide concrete justifications!

When it comes to major components replacements, it’s worth noting that the original engine calibrations are already optimized specifically for your make, displacement, and emissions. This means that you won’t need to spend extra money on a costly peterbilt ECM reflash. With this in mind, you can rest assured that you’ll be able to keep your wallet safe from any sneaky stealership money grabs.


In my decades and over a million miles behind the wheel, engine derates stand out as one of the most disruptive issues facing peterbilt owner-operators and fleet managers alike trying to keep trucks rolling down highways profitably.

Understanding what causes power restrictions, interpreting trouble codes, methodically fixing underlying issues, properly resetting derate counters, and focusing on preventative maintenance provides the blueprint to maximize Peterbilt uptime and enhance Peterbilt productivity.

Hopefully, this real-world, experience-based overview gives you confidence next time your Peterbilt throws a derate code, forcing you curbside. Don’t panic and don’t allow dealerships to take advantage. Trust the Peterbilt derate troubleshooting process outlined above, combined with your own truck knowledge, and you’ll have things running smoothly faster than you expect.

Let me know if you run into any other Peterbilt derate questions out on those long lonesome highways, emblematic of the Peterbilt long haul experience! Safe travels, my friends.

Leave a comment

About Mohammad Sameer

My name is Mohammad Sameer and I have over 3 years of hands-on experience repairing cars, motorcycles, and trucks. Ever since I operated on my first engine in 2018, I’ve been passionate about all things automotive. In 2021, I launched my blog “Motoring Mastery” to share my knowledge with car enthusiasts and DIY mechanics.