ECU Fault Codes: How to Tell What’s Wrong with Your ECU

When a car’s Engine Control Unit (ECU) senses a problem or malfunctioning within the vehicle, it triggers an ECU fault code that will turn on the check engine light. If your check engine light comes on, the ECU fault code can tell you exactly what problem your vehicle is experiencing.

ECU fault codes, also known as diagnostic trouble codes (DTC) are part of the on-board diagnostic (OBD II) system in your car. These 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.

How to Tell What’s Wrong Using ECU Fault Codes

ECU fault codes include a letter followed by four numbers. The first letter (either P, B, C, or U), informs the reader of the system being affected by the problem. In most cases, you will see a “P” that indicates a powertain issue, which includes a vehicle’s engine, emission, and transmission systems. “B” codes signal body issues, such as problems with climate control, lighting, or airbags. “C” codes point to chassis problems, including braking, steering, or suspension issues, and “U” codes indicate network communication errors, such as wiring problems.

The second digit in an ECU fault code will usually be “0” or “1.” A “0” tells you that it is a generic ECU fault code, while a “1” indicates that the code is specific to your car manufacturer.

The next digit explains what the ECU trouble code pertains to. The following numbers indicate problems within specific systems or controls within your vehicle.

1. Emission Management (Fuel or Air Metering)
2. Injector Circuit (Fuel or Air Metering)
3. Ignition or Misfire
4. Auxiliary Emission Controls
5. Vehicle Speed Control and Idle Speed Control
6. Computer and Output Circuit
7. Transmission
8. Transmission

The last two digits in the ECU fault code tell the exact problem that your car is experiencing.

Common ECU Fault Codes

Despite the amount of diagnostic trouble codes, some vehicle problems are seen more often than others, causing certain ECU fault codes to be more common. The following ECU fault codes are triggered by commonly seen vehicle errors.


These codes represent problems with engines misfiring. The last two digits specify which engine cylinder is experiencing the misfires, or if the misfires are not contained to one cylinder. Misfires can occur for many reasons, so it is important to have your car looked at by an auto technician to determine which parts are faulty and in need of a fix.

P0171, P0174

These codes represent fuel trim trouble. These codes are triggered when the fuel/air ratio includes too much air or too little fuel, giving you a lean fuel mixture. This problem can be caused by leaky valves, dirty sensors or fuel injectors, or low fuel pressure due to pumps or the fuel pressure regulator.

P0411, P0440, P0442, P0446, P0455

Problems with the Evaporative Emission Control (EVAP) System will trigger these codes if fuel vapor is failing to escape from the fuel tank. These issues are commonly related to leaks, and experienced technicians can find the exact location of the leak to find the part in need of repair.

P0420, P0430

A catalytic converter code usually is triggered when the converter needs replaced. If your vehicle senses leaking coolant or burning oil, these ECU fault codes will be triggered to indicate that the oxygen sensors have noticed an inefficiency with the catalytic converter.

P0133, P0135, P0141

These oxygen sensor codes are set off with the malfunctioning of the oxygen sensor itself, or with the oxygen heater circuit that heats the O2 sensor upon starting a car. If either of these sensors experience problems, your vehicle may experience a lower fuel economy due to a problem with the fuel mixture or air/fuel ratio.

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