How to Read a Check-Engine Code (and What to Do Next)

A check-engine light means the engine computer saw a fault. A flashing light means an active misfire that can damage the catalytic converter. Reduce load and get it checked soon.

Step by step

With the key off, plug the scanner into the OBD2 port.
Turn the key to ON (engine off), or start the engine if the tool says to.
Read the stored, pending, and permanent codes. Write them down along with the freeze frame data before you clear anything.

Decoding the format

Example: P0301. The first letter is the system: P = powertrain, B = body, C = chassis, U = network. The first digit is 0 for a generic code or 1 for manufacturer-specific. The remaining digits narrow it down. P030x codes are misfires, and the last digit is the cylinder number (P0300 = random).

Common generic codes

P0420: catalyst efficiency below threshold (bank 1). Check for exhaust leaks and O2 sensor function before you buy a converter.
P0171/P0174: system too lean. Vacuum leaks are a common cause. Check the fuel trims.
P0455/P0456: EVAP leak. Check the gas cap first.
P0128: coolant below thermostat regulating temperature, often a thermostat stuck open.

Code ≠ part

A code tells you which test failed, not which part to replace. Look up known fixes for your exact vehicle, check freeze frame, and verify with live data or a multimeter before you buy parts.

Need a scanner? See our OBD2 scanner guide.