OBD-II trouble codes
The standard codes a scan tool reads when the check engine light comes on, and what each one names.
An OBD-II trouble code is the five-character code a car's on-board diagnostic system stores when it finds a fault, usually as it lights the check engine light. The codes and their names are set by SAE J2012, so a code such as P0420 means the same thing on any car that uses the generic set.
Catalytic converter 2 symbols
Misfire 7 symbols
Fuel and air 8 symbols
- P0171: System too lean (bank 1)
- P0174: System too lean (bank 2)
- P0172: System too rich (bank 1)
- P0175: System too rich (bank 2)
- P0101: Mass or volume air flow circuit range/performance
- P0102: Mass or volume air flow circuit low input
- P0106: Manifold absolute pressure/barometric pressure circuit range/performance
- P0113: Intake air temperature sensor 1 circuit high
Evaporative emissions 8 symbols
- P0456: Evaporative emission system leak detected (very small leak)
- P0442: Evaporative emission system leak detected (small leak)
- P0455: Evaporative emission system leak detected (large leak)
- P0440: Evaporative emission system
- P0441: Evaporative emission system incorrect purge flow
- P0443: Evaporative emission system purge control valve circuit
- P0446: Evaporative emission system vent control circuit
- P0449: Evaporative emission system vent valve/solenoid circuit
Oxygen sensors 1 symbol
Other engine systems 5 symbols
How a code is built
SAE J2012 describes each code as a letter and a digit followed by three more digits. The letter gives the area of the car: B for body, C for chassis, P for powertrain (the engine and transmission) and U for the network that links the car's computers. After P, a 0 marks a code whose name is set by the standard, and a 1 a code whose meaning each manufacturer sets for itself; P2 codes are set by the standard too. A generic list of OBD-II codes sets out the P0 codes the same way.
Where the codes are read
The federal rule on on-board diagnostics requires the stored codes to be available through the car's standardized diagnostic connector, and access to them to be unrestricted: it may not need any code or device that only the manufacturer supplies.
Sources
- SAE J2012: Diagnostic Trouble Code Definitions (2002), SAE J2012, April 2002 (technically equivalent to ISO/DIS 15031-6), SAE International; published by Public.Resource.Org on the Internet Archive, § 5, Format structure
- Generic OBD-II Trouble Codes (scanned reference list), Internet Archive (item generic-and-honda-trouble-codes), Title page
- 40 CFR 86.1806-05, On-board diagnostics (2012 edition), 40 CFR 86.1806-05, US Government Publishing Office (Code of Federal Regulations), § 86.1806-05(h)
Last reviewed September 27, 2026 by Rory Hansen. How we research
Related products on Amazon
As an Amazon Associate we earn from qualifying purchases made through these links. The pictures are our own drawings of the kind of product, not a particular one, and nothing here is a recommendation.