What is OBD II codes?
What the system does
OBD II stands for second-generation On-Board Diagnostics. The engine control module and other computers constantly monitor sensors and systems. When a reading falls outside its expected range, or a system fails a self-test, the computer stores a code and may turn on the check engine light.
The same five-character format is used across most makes, which is why a basic scanner can read codes from many different cars. Europe uses a similar system called EOBD. Exact dates and coverage vary by country and vehicle type.
How a code is structured
Every code starts with one letter that identifies the system: P for powertrain, B for body, C for chassis, and U for network. The second character tells you whether the code is generic (0) or manufacturer-specific (1). The last three characters identify the specific fault within that system.
- P: engine, transmission, and emissions
- B: body systems such as airbags and power seats
- C: chassis systems such as ABS and steering
- U: communication between control modules
What to do with a code
Plug a scanner into the diagnostic port, usually under the dashboard on the driver's side, and record every code along with any freeze frame data. Then look up the code for your specific make, model, and year. Treat the code as the starting point of a test, not the final answer.
Common mistakes
- Replacing the part named in a code without testing it first.
- Clearing the code and assuming the problem is solved without finding the cause.
