Why Is My Check Engine Light On? 5 Possible Causes

Few things are more frustrating than seeing your check engine light pop on while driving your heavy-duty truck or commercial bus. That small, glowing indicator can mean anything from a minor issue to a major repair waiting to happen. But don’t panic just yet! 

Today, we’ll go through five of the most common reasons why your check engine light might be on—so you can diagnose the problem and get back on the road in no time!

1. Faulty Diesel Particulate Filter (DPF) or Regeneration Issues

Your engine’s diesel particulate filter (DPF) captures and reduces soot emissions. Over time, this filter needs to regenerate by burning off the accumulated soot. If something interferes with this process, the DPF can clog, triggering the check engine light.

Signs of a DPF Problem

• Loss of power when accelerating
• Increased fuel consumption
• A burning smell from the exhaust
• Your truck is going into limp mode

What to Do

Try a forced regeneration if your system allows it. If that doesn’t work, you may need to have the DPF professionally cleaned or replaced. Regular highway driving at high RPMs can also help prevent DPF clogging in the future.

2. Fuel System Issues (Low Fuel Pressure or Bad Injectors)

Your fuel system delivers the precise amount of diesel fuel to your engine. Any disruption—such as a clogged fuel filter, failing fuel pump, or faulty injectors—can cause low fuel pressure, triggering the check engine light.

Common Symptoms of Fuel System Problems

• Hard starts or no start
• Rough idling or engine misfires
• Loss of power under load
• Poor fuel economy

What to Do

Start by checking your fuel filter. A clogged filter is one of the most common culprits. If replacing it doesn’t fix the issue, you may need a fuel pressure test to diagnose problems with the injectors or fuel pump.

3. EGR Valve Malfunction (Exhaust Gas Recirculation System)

Your exhaust gas recirculation (EGR) valve helps lower emissions by recirculating a portion of exhaust gases back into the engine. If it becomes clogged with soot or malfunctions, your check engine light will come on.

Signs of a Faulty EGR Valve

• Rough idling or stalling
• Excessive black smoke from the exhaust
• Decreased fuel efficiency
• A noticeable loss of power

What to Do

A carbon-cleaning service can often clear minor buildup in your EGR system. If that doesn’t work, you may need to replace the EGR valve entirely.

4. Turbocharger Problems (Boost Pressure Issues)

A properly functioning turbocharger helps your heavy-duty diesel engine produce more power efficiently. But if your turbo boost levels aren’t where they should be, your engine control module (ECM) will detect it and turn on the check engine light.

Signs of Turbocharger Issues

• Loss of power, especially under load
• Excessive black smoke from the exhaust
• A whining or siren-like noise from the turbo
• Oil leaks near the turbo housing

What to Do

Start by checking for boost leaks in your intercooler piping and intake hoses. A cracked or loose hose can cause a significant drop in boost pressure. If the turbo itself is failing, it may need rebuilding or replacement.

5. DEF System Malfunction (Diesel Exhaust Fluid Issues)

If your truck or bus runs on Selective Catalytic Reduction (SCR) technology, it relies on Diesel Exhaust Fluid (DEF) to reduce emissions. Any issue with the DEF pump, DEF sensors, or DEF injector can trigger a check engine light and even put your vehicle into limp mode.

Signs of a DEF System Problem

• Check engine light accompanied by a DEF warning message
• Reduced power mode (limp mode)
• Crystallized DEF buildup near the tank or injector
• Strong ammonia smell from the exhaust

What to Do

Make sure your DEF tank is full and that you're using high-quality DEF fluid. If the issue persists, a scan tool can identify whether it’s a DEF sensor failure, pump issue, or injector clog.

What to Do When Your Check Engine Light Comes On

Seeing that check engine light can be stressful, but here’s what you should do:

1. Check for Obvious Issues

Look for loose connections, leaks, or low fluid levels. Sometimes, a simple fix—like tightening a fuel cap or topping off DEF—can turn the light off.

2. Use a Diagnostic Scanner

A heavy-duty truck diagnostic scanner can pull trouble codes to help pinpoint the problem. If you don’t have one, many repair shops can scan your codes for free.

3. Don’t Ignore It!

Even if your truck or bus seems to be running fine, ignoring the check engine light can lead to bigger (and more expensive) repairs down the road. Always consult a professional heavy-duty repair shop if you’re ever in doubt.

Recap

A check engine light in your heavy-duty diesel truck or bus isn’t something to ignore, but it doesn’t always mean disaster. The most common culprits include DPF issues, fuel system failures, EGR malfunctions, turbo problems, and DEF system failures. Diagnosing and addressing these problems early can avoid costly breakdowns and keep your rig running smoothly.