Check Engine Light On? Here are the Five Most Common PCM/ECU Fault Codes.

check engine light codes

When your vehicle’s Engine Control Unit (ECU) senses a problem or malfunction within the vehicle, it will trigger an ECU fault code, which then will prompt the check engine light to illuminate.

ECU fault codes, also known as diagnostic trouble codes (DTC), are part of the on-board diagnostic (OBD II) system in your car. These fault codes provide information on which systems are malfunctioning, where the problem is located, and the exact issue that is causing your check engine light to remain illuminated.

Most fault codes will not result in a full ECU replacement/repair, however these codes are still indicators of a problem with your vehicle that can cause additional damage to your vehicle if not repaired.

Common ECU Fault Codes

While there are many reasons why and ECU would trigger a diagnostic fault code, some codes will appear more frequently than others. The following are some ECU failure codes triggered by common vehicle errors


These codes appear when an engine misfire has been detected. The last two digits of the code will specify which engine cylinder is affected, or if the misfires are not contained to one engine cylinder. These errors can occur for a variety of different reasons, so it’s important to have your mechanic look at your vehicle to determine where the malfunction occurred.

P0171, P0174

These codes apply to your vehicles air/fuel ratio and is triggered when the air/fuel mixture deviates from the normal; either running too lean or too rich. This problem may be caused by a few items, but commonly is due to leaky valves, dirty sensors, dirty fuel injector lines, or low fuel pressure caused by a malfunctioning fuel pump or fuel pressure regulator.

P0411, P0440, P0442, P0446, P0455

The above codes may be triggered when a problem is detected within the Evaporative Emissions Control Systems  (EVAP). Commonly these are related to leaks within the EVAP, and a qualified mechanic should be able to identify the exact location of the leak.

P0420, P0430

The above two codes are commonly triggered when the catalytic converter needs to be replaced or is operating inefficiently. If your vehicle senses a leak in the cooling system, or burning oil, these ECU fault codes may be triggered to indicate the oxygen sensors have registered a problem with the catalytic converter.

P0133, P0135, P0141

The above three codes are triggered when a malfunction with the oxygen sensor or the oxygen heating circuit is registered. A malfunction with either of these sensors will generally result in lower fuel economy.

Detecting ECU Fault Codes

While ECU Repair Now is a leader in ECU module and automotive electronic repairs, we recommend first having a qualified mechanic inspect your vehicle and properly diagnose the issue. However, if a major failure with the ECU has caused the fault code, one of our qualified electrical technicians would be happy to discuss your repair/replacement options.