Check engine light on: Codes, Causes & Fixes

P0174 Powertrain The cylinder bank without cylinder 1 is running lean even with the maximum fuel correction… Moderate P0420 Powertrain The bank 1 catalytic converter no longer stores enough oxygen to clean the exhaust properly. Low P0101 Powertrain The MAF sensor's airflow reading doesn't match what the engine computer expects… Low P0113 Powertrain The intake air temperature signal is stuck at maximum voltage, which usually means an open… Low P0135 Powertrain The heater inside the upstream oxygen sensor on bank 1 isn't working or its circuit is faulty. Low P0171 Powertrain The engine computer is adding the maximum extra fuel it's allowed on bank 1 and still sees… Moderate

P0402 Powertrain More exhaust is being recirculated than the ECU asked for, usually because the EGR valve won't… Moderate P0403 Powertrain The ECU found an electrical fault in the circuit that drives the EGR solenoid or electric EGR… Low P0404 Powertrain The EGR valve isn't moving to the position the ECU commands, or its position report doesn't… Low P0405 Powertrain The EGR position or pressure sensor signal is stuck near zero volts, below its normal operating… Low P0406 Powertrain The EGR feedback sensor is reporting close to 5 volts or more, above anything a working sensor… Low P0409 Powertrain The ECM lost a usable signal from the EGR position or pressure sensor – often an open… Low P0410 Powertrain The cold-start air pump system that helps heat the catalytic converter didn't pass its… Low P0411 Powertrain The air pump runs at cold start, but the right amount of air isn't reaching the exhaust. Low P0412 Powertrain The ECU found an open or short in the circuit of the valve that routes air from the pump into… Low P0418 Powertrain The ECU sees a fault in the relay circuit that switches the secondary air pump on and off. Low P0420 Powertrain The bank 1 catalytic converter no longer stores enough oxygen to clean the exhaust properly. Low P0421 Powertrain The close-coupled warm-up catalyst on bank 1, right behind the exhaust manifold, isn't working… Low P0422 Powertrain The main underfloor catalytic converter on bank 1 no longer stores enough oxygen to pass… Low P0430 Powertrain The bank 2 converter on a V6, V8 or split-exhaust engine isn't cleaning the exhaust well enough. Low P0431 Powertrain On a V6, V8 or flat engine, the warm-up converter on the side without cylinder #1 is no longer… Low P0440 Powertrain Catch-all EVAP code: the system failed its self-check. Start with the gas cap, then check hoses… Minor P0441 Powertrain The ECU opened the purge valve but saw the wrong flow: too little, too much, or at the wrong… Low P0442 Powertrain The EVAP system leaks through an opening of about 0.040 inch — a cracked hose, loose joint… Minor P0443 Powertrain The ECU detected an electrical fault in the purge solenoid circuit: open, shorted or a bad… Low P0444 Powertrain The purge valve circuit is broken somewhere: unplugged connector, open coil, broken wire or no… Minor P0446 Powertrain The canister vent valve isn't doing what the ECU commands — electrically or because the vent… Minor P0447 Powertrain The canister vent solenoid circuit is open, so the ECU can't close the vent to run its leak… Minor P0448 Powertrain The canister vent solenoid circuit has a short, which can hold the vent shut and make refueling… Low P0449 Powertrain Electrical fault on the canister vent solenoid circuit, a staple on GM trucks with the solenoid… Low P0450 Powertrain The fuel tank (EVAP) pressure sensor failed a self-check, so the ECU can't trust its leak tests. Minor P0451 Powertrain The tank pressure reading is electrically fine but doesn't make sense: offset, stuck, noisy… Minor P0452 Powertrain The tank pressure sensor signal is stuck near 0 volts, lower than the sensor can physically… Minor P0453 Powertrain Tank pressure signal pinned near the top of its range: lost ground, short to voltage or real… Minor P0455 Powertrain The fuel vapor system can't hold vacuum at all — check the gas cap first, then hoses and valves. Low P0456 Powertrain The EVAP system has a tiny leak, about 0.020 inch — the hardest EVAP leak to find, and often… Minor