How to Read a Check Engine Code (and What It Doesn't Tell You)

How codes are structured

P = powertrain, B = body, C = chassis, U = network. The next digit is 0 for generic or 1 for manufacturer-specific. The last three digits identify the fault. So P0301 is a generic powertrain code for a cylinder 1 misfire.

The ones we see most

• P0420/P0430 – Catalyst efficiency (often a sensor or leak, not the converter)
• P0171/P0174 – System too lean (vacuum leak, MAF, fuel delivery)
• P0300–P0308 – Misfire (plugs, coils, injectors, compression)
• P0442/P0455/P0456 – EVAP leak (gas cap, purge valve, hose)
• P0128 – Coolant temp below thermostat (stuck-open thermostat)
• P0135/P0141 – O2 sensor heater circuit (sensor)
• P0700 – Transmission control fault (check the transmission module for the real code)

What the code doesn't tell you

It tells you which test the computer failed, not which part broke. A lean code can be a cracked intake hose, a dirty airflow sensor, a weak fuel pump, or a bad injector. That's four different repairs at four different prices. Diagnosis — live data, freeze frame, testing the component — is what turns a code into a fix.

Pending, stored, and permanent

A pending code has happened once and hasn't triggered the light yet. A stored code turned the light on. A permanent code can't be cleared with a scan tool — only by the car passing the test again. If you're buying a used car and see permanent codes, ask questions.

Readiness monitors

After codes are cleared, the car runs self-tests over several drives. Until those 'monitors' are complete, an emissions inspection will fail. Clearing the light the morning of inspection doesn't work.