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Check engine light on: Codes, Causes & Fixes

A steady check engine light means the engine computer has stored a fault that affects emissions or engine management. In most cases you can keep driving normally for now, but have the code read soon – many auto parts stores and any OBD-II scanner can do this. If the light came on right after refueling, check that the gas cap is tight; loose caps are a classic trigger for EVAP codes.

558Codes
2.7Average severity
12%High or critical
18%DIY-friendly
Severity distributionSeverity distribution

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P0135 Powertrain The heater inside the upstream oxygen sensor on bank 1 isn't working or its circuit is faulty. Low P0174 Powertrain The cylinder bank without cylinder 1 is running lean even with the maximum fuel correction… Moderate P0300 Powertrain The engine computer sees misfires that jump between cylinders or hit several cylinders at once. High P0420 Powertrain The bank 1 catalytic converter no longer stores enough oxygen to clean the exhaust properly. Low P0430 Powertrain The bank 2 converter on a V6, V8 or split-exhaust engine isn't cleaning the exhaust well enough. Low P0101 Powertrain The MAF sensor's airflow reading doesn't match what the engine computer expects… Low

All codes (68)

All Severity 3+ Severity 4+
U0101 Network The engine computer and other modules stopped hearing from the transmission control module. High U0105 Network The PCM lost contact with the separate module that fires the injectors, common on diesel… High U0107 Network The engine computer can't reach the module that drives the electronic throttle, so power is cut… High U0109 Network The PCM lost contact with the module that controls fuel pump speed; stalling or no-start can… High U0110 Network The hybrid/EV system lost contact with the module that runs electric drive motor A. Critical U0115 Network Modules lost contact with the second engine computer on vehicles that use two of them. High U0404 Network The electronic shifter reports a lever position the transmission can't trust, so Park may not… High U0418 Network Brake-control data such as pedal position or brake-assist status looks implausible to another… High

Symptoms

Loss of power 214Rough idle 184Limp mode 165Poor fuel economy 146Engine stalling 133Car won't start 123Harsh or slipping shifts 80ABS / traction light on 65Fuel smell 34Check engine light flashing 22