Understanding DTC Codes: P, B, C, U Explained
How to Read a DTC Code
Every OBD-II diagnostic trouble code follows the same 5-character format: one letter + four digits. Here's what each position means:
Position 1: The System Letter
| Letter | System | Examples |
|---|---|---|
| P | Powertrain | Engine, transmission, fuel, ignition |
| B | Body | Airbags, A/C, lighting, seats |
| C | Chassis | ABS, steering, suspension, brakes |
| U | Network/Undefined | CAN bus, module communication |
Position 2: Generic vs. Manufacturer-Specific
- 0 = Generic (SAE standard — same meaning across all makes)
- 1 = Manufacturer-specific (meaning varies by brand)
Position 3: Subsystem
For P-codes, the third digit indicates the subsystem:
- 1, 2 = Fuel and air metering
- 3 = Ignition system / misfire
- 4 = Emissions controls
- 5 = Vehicle speed / idle control
- 6 = Computer / output circuits
- 7, 8 = Transmission
Positions 4-5: Specific Fault
The last two digits identify the exact fault. For example, in P0420: "20" points to catalyst system efficiency below threshold (Bank 1).
Example Breakdown
Let's decode P0171:
- P = Powertrain
- 0 = Generic (standard across all makes)
- 1 = Fuel and air metering
- 71 = System too lean (Bank 1)
Translation: The engine computer detected that Bank 1 is running too lean — too much air or too little fuel.
Look up any specific code on FixCarCodes.com for detailed diagnosis and repair steps.