What does your check-engine code mean?

The P0112 code means the intake air temperature (IAT) sensor circuit reported voltage too low — a grounded short — so the engine control module (ECM) reads the incoming air as extremely hot and leans out the fuel mixture for heat that isn't there.

The P0113 code means the intake air temperature (IAT) sensor circuit reported an impossibly high voltage — an open circuit — so the computer reads the incoming air as −40° and starts fueling for arctic weather it isn't getting.

The P0117 code means the circuit watching the engine coolant reported voltage too low — a short — so the engine control module (ECM) reads the coolant as impossibly hot, skips cold-start enrichment, runs the cooling fans continuously, and can no longer tell a real overheat from a lie.

The P0118 code means the circuit watching the temperature of the engine coolant reported voltage too high — an open circuit — so the engine control module (ECM) reads the coolant as −40°, fuels the engine as if it were frozen, and can no longer see real trouble coming.

The engine isn't reaching its normal operating temperature in the time the ECU expects — usually because the thermostat is stuck open and over-cooling the engine.

The engine is getting too much air for the fuel it's burning. The ECU added fuel to compensate and ran out of room — so it set a lean code.

The engine control unit detected misfires it couldn't pin to a single cylinder — combustion is failing in more than one place.

The engine is misfiring in one specific place: cylinder 1. Combustion there is failing often enough for the ECU to flag it.

The P0402 code means the computer detected exhaust gas recirculation flow excessive — more exhaust re-entering the engine than commanded. Either the EGR valve is genuinely stuck open, usually by carbon buildup, or a flow sensor is exaggerating. At idle, that extra inert gas makes the engine stumble and stall.

Based on the oxygen sensor after the catalytic converter, the ECU thinks the cat isn't cleaning the exhaust as well as it should.

The P0446 code means the computer has detected an issue in the evaporative emission control system vent control circuit — the breathing path through the vent valve and charcoal canister is blocked, stuck, or not responding, so the fuel tank can't vent properly.

The P0449 code means the engine computer found a circuit malfunction in the evaporative emission system's vent valve solenoid — an open circuit or short in the coil, the wiring, or the driver. It's an electrical fault, not a fuel-vapor leak: the gas cap is innocent on this one.

The evaporative emission control (EVAP) system failed its pressure test — fuel vapor is escaping through what the ECU calls a large leak (a 'gross leak,' in older terms).

The P0456 code means the evaporative emission (EVAP) system detected a very small leak — fuel vapor escaping through an opening around 0.020 inches, thinner than a pencil lead.