Most Common OBD2 Diagnostic Codes
Top-searched engine trouble codes responsible for 80% of Check Engine Lights
Catalyst System Efficiency Below Threshold (Bank 1)
Driving with P0420 will not cause immediate catastrophic failure, but fuel economy will drop and your vehicle will fail annual state/provincial emissions inspection. If raw fuel is entering the exhaust, prolonged driving may melt the catalytic converter honeycomb.
System Too Lean (Bank 1)
A lean condition means too much air and not enough fuel in the cylinders. Driving long distances lean causes combustion temperatures to spike, which can lead to burnt exhaust valves, melted pistons, and catalytic converter damage.
Random or Multiple Cylinder Misfire Detected
If the Check Engine light is SOLID, you can drive gently to a safe place. If the Check Engine light is FLASHING, unburnt raw fuel is dumping into the exhaust and will overheat and destroy your catalytic converter in less than 5 miles, costing over $1,500 in damages.
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Code P0128 means the engine is taking too long to warm up to operating temperature (usually 185?F - 205?F / 85?C - 95?C). While you won't blow a head gasket from overheating, the ECU will remain in 'Open Loop' mode, running rich and burning 15% more fuel.
Evaporative Emission System Leak Detected (Gross Leak / Large Leak)
Code P0455 is an emissions-only fault. It means fuel tank vapors are escaping into the atmosphere rather than being routed into the engine to burn. The vehicle will run normally with zero loss of power, but it will fail state emissions inspection.
O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
Upstream O2 sensors require temperatures above 600?F to measure oxygen accurately. Modern O2 sensors have internal electric heating elements. When the heater circuit fails, the ECU must wait for exhaust heat to warm the sensor, running in an inefficient rich mode during warm-up.
Idle Air Control System RPM Higher Than Expected
Code P0507 triggers when the engine idle speed is 200+ RPM higher than the target speed stored in the ECU (typically idle should be 650-750 RPM, but stays at 900-1200 RPM). You will need to press the brake pedal harder at stoplights.
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