An OBD-II trouble code is the five-character label a vehicle stores when it detects a fault. Reading the code with a scanner points you at the system at fault. This reference explains the format and lists the codes drivers see most.
How a code is structured
Take P0171 as an example: P is powertrain, 0 is a generic SAE code, 1 is the fuel and air subsystem, and 71 is the specific fault (system too lean).
| Position | Meaning |
|---|---|
| 1st — letter | System: P powertrain, B body, C chassis, U network |
| 2nd — digit | 0 generic (SAE standard), 1 manufacturer-specific |
| 3rd — digit | Subsystem: 1-2 fuel/air, 3 ignition, 4 emissions, 5 speed/idle, 6 computer, 7-8 transmission |
| 4th and 5th | The specific fault number |
Common trouble codes
| Code | Meaning | Common causes |
|---|---|---|
| P0101 | MAF sensor range / performance | Dirty MAF sensor, intake leak |
| P0113 | Intake air temp sensor high input | IAT sensor, wiring |
| P0128 | Coolant below thermostat temp | Stuck-open thermostat |
| P0131 | O2 sensor circuit low (B1S1) | Oxygen sensor, exhaust leak |
| P0171 | System too lean (Bank 1) | Vacuum leak, weak fuel pump, dirty MAF |
| P0172 | System too rich (Bank 1) | Leaking injector, high fuel pressure |
| P0300 | Random / multiple misfire | Plugs, coils, fuel, vacuum leak |
| P0301-P0308 | Cylinder 1 to 8 misfire | Plug, coil, or injector on that cylinder |
| P0325 | Knock sensor circuit | Knock sensor, wiring |
| P0335 | Crankshaft position sensor | Sensor, wiring, reluctor ring |
| P0340 | Camshaft position sensor | Sensor, timing, wiring |
| P0401 | EGR flow insufficient | Clogged EGR valve or passages |
| P0420 | Catalyst efficiency low (Bank 1) | Failing catalytic converter, O2 sensor |
| P0430 | Catalyst efficiency low (Bank 2) | Failing converter, O2 sensor (bank 2) |
| P0440 | EVAP system malfunction | Loose or faulty gas cap, EVAP leak |
| P0442 | EVAP small leak | Gas cap, small hose leak |
| P0455 | EVAP large leak | Gas cap, large hose or canister leak |
| P0500 | Vehicle speed sensor | Speed sensor, wiring |
| P0506 | Idle RPM lower than expected | Dirty throttle body, idle air control |
| P0700 | Transmission control system | Stored transmission fault, check TCM |
| P0720 | Output speed sensor circuit | Speed sensor, wiring |
A code names the symptom, not always the part. P0171 (too lean) is most often a vacuum leak, not a bad sensor. Diagnose the cause before replacing parts.
Generic SAE definitions; manufacturer-specific (P1xxx) codes vary by brand. Always confirm against service information for your vehicle.
See the Understanding OBD-II Codes guide and the Dashboard Warning Light Guide.