How to read an OBD2 code
Every OBD2 trouble code has five characters, and each position means something. Once you know the pattern, a code like P0171 already tells you the system and whether the meaning is the same on every car.
First character: the system
| Letter | System |
|---|---|
| P | Powertrain (engine and transmission) |
| B | Body |
| C | Chassis |
| U | Network communication |
Second character: generic or manufacturer
A 0 in the second position means a generic code. Generic code titles are standardized by SAE (J2012) and are meant to mean the same thing on every OBD2 vehicle. P2 codes are generic too. A 1 means a manufacturer specific code, whose meaning depends on the carmaker.
Even for generic codes, a few manufacturers vary from the standard definitions, so confirm the meaning for your vehicle before buying parts.
Third character: the subsystem (P0 codes)
| Third digit | Subsystem | Examples on this site |
|---|---|---|
| 0, 1, 2 | Fuel and air metering | P0101, P0171, P0299 |
| 3 | Ignition system or misfire | P0300, P0301, P0335 |
| 4 | Auxiliary emission controls | P0420, P0442, P0401 |
| 5 | Vehicle speed and idle control | P0500, P0505 |
| 6 | Computer and output circuits | Not covered on this site yet |
| 7, 8 | Transmission | P0700 |
Last two characters: the exact fault
The last two characters identify the specific fault within that subsystem. Newer codes can also use letters in these positions, for example P042A, because the number of possible faults has grown.
Worked example: P0171
- P: powertrain.
- 0: generic SAE code.
- 1: fuel and air metering.
- 71: the specific fault, System Too Lean (Bank 1).
The code tells you which self test failed, not which part to replace. See the P0171 guide for the usual causes and checks.
Look up codes with OBD Master Pro
OBD Master Pro shows code descriptions from generic and brand databases, and you can search a code without connecting to a car. Read every code before clearing anything, since several codes together often point to one cause.
Try it on your own car
OBD Master Pro turns your Android phone and a generic ELM327 adapter into a code reader: read and clear codes, view freeze frame and live data, and check emissions readiness.
Keep reading
Related guides: Check engine light on: what to do · Stored, pending and permanent codes · OBD2 readiness monitors explained · ELM327 adapter not connecting · How to clear the check engine light · What is fuel trim · OBD2 live data and PIDs · OBD2 port pinout · What is a U code? · OBD2 glossary
Common codes: P0171 System Too Lean · P0420 Catalyst Efficiency · P0300 Random Misfire · P0455 EVAP Large Leak
All OBD2 guides · OBD2 by car brand · OBD2 trouble code guides
Sources
- SAE J2012: Diagnostic Trouble Code Definitions
- Maryland Department of the Environment, Vehicle Emissions Inspection Program: Decoding DTCs
- OBD-codes.com: OBD2 codes explained
- OBD-codes.com: OBD-II trouble code lists (generic, manufacturer, body, chassis and network codes)
- Clemson University Vehicular Electronics Laboratory: On-Board Diagnostic Systems
- OBD Master Pro on Google Play (app features, supported connections and protocols)
Published . Updated . General information only. Details generally vary by model, year and market, so always follow the owner's manual and service information for your vehicle.