Reading OBD-II trouble codes
A diagnostic trouble code has five characters — for example P0420:
P = powertrain · 0 = ISO/SAE controlled · 4 = subsystem (here, emissions) · 20 = the specific fault.
This tool currently covers imported P0 powertrain definitions. Generic SAE-controlled powertrain ranges also include P2xxx and P34xx-P39xx; P1xxx and P30xx-P33xx are manufacturer-controlled. A stored code points at a circuit or system, not always a specific broken part — it's the start of diagnosis, not the end of it.
FAQ
Does a code tell me exactly what to replace?
Rarely. A code like "O2 sensor circuit low" flags the circuit, but the cause could be the sensor, the wiring, a connector, or even an exhaust leak fooling it. Use the code to aim your diagnosis — check live data and wiring before throwing parts at it.
Why isn't my P1, P2 or P3 code here?
This tool currently covers the generic P0 powertrain range. P2 and P34-P39 are also generic SAE-controlled ranges and are on the roadmap. P1 and P30-P33 are manufacturer-controlled, so the same code can mean different things by make — those need make-specific data.
Can I clear a code myself?
A scan tool can clear codes, but if the underlying fault is still present the light comes back — and clearing readiness monitors can fail an emissions test until they re-run. Fix the cause first, then clear.