The OBD-II fault codes are made of a letter following by four numbers. For example P115C (BMW: Low Air Mass Compared To Model).
The letter can be one of:
| Letter | Description |
|---|---|
B |
Body |
C |
Chassis |
P |
Powertrain |
U |
Network Communication |
In the 2nd position is a number:
| Num | Description |
|---|---|
0 |
Standardized Code (ISO/SAE) |
1 |
Manufactureur Specific Code |
In the 3rd position the number is the affected subsystem:
| Num | Subystem |
|---|---|
1 |
Secondary air injection |
2 |
Fuel |
3 |
Ignition |
4 |
Exhaust monitoring |
5 |
Idle speed control or Cruise control |
6 |
Input / Output signal from ECU |
7 |
Transmission system |
8 |
Transmission system |
The 4th and 5th numbers are code for the specific issue.
Thus P115C is a constructor specific code for the Powertrain > Secondary Air Injection system. The specific code being 5C.
For the PIDS see https://en.wikipedia.org/wiki/OBD-II_PIDs