OBD2 Diagnostic Trouble Codes (DTC) Reference
Every OBD2 code follows the same format: one letter + four digits. The letter tells you which system is affected. The first digit tells you if it’s generic (0) or manufacturer-specific (1).
How to Read an OBD2 Code
P0420 = Powertrain | 0 Generic | 4 Emissions | 20 Specific fault
Code types: P = Powertrain (engine/transmission) | B = Body | C = Chassis | U = Network/Communication
Most Common Engine Codes (P0xxx)
| Code | Meaning | Common Cause | Drive? |
|---|---|---|---|
| P0101 | Mass Air Flow Sensor Range/Performance | Dirty MAF sensor, vacuum leak, dirty air filter | Yes — reduced power |
| P0171 | System Too Lean (Bank 1) | Vacuum leak, dirty MAF, weak fuel pump, clogged filter | Yes — may run rough |
| P0172 | System Too Rich (Bank 1) | Leaking injector, bad O2 sensor, high fuel pressure | Yes — poor MPG |
| P0300 | Random/Multiple Cylinder Misfire | Worn plugs, vacuum leak, low fuel pressure, bad coils | Yes — fix soon |
| P0301 | Cylinder 1 Misfire | Spark plug, ignition coil, fuel injector on cyl 1 | Yes — fix soon |
| P0302 | Cylinder 2 Misfire | Spark plug, ignition coil, fuel injector on cyl 2 | Yes — fix soon |
| P0303 | Cylinder 3 Misfire | Spark plug, ignition coil, fuel injector on cyl 3 | Yes — fix soon |
| P0304 | Cylinder 4 Misfire | Spark plug, ignition coil, fuel injector on cyl 4 | Yes — fix soon |
| P0325 | Knock Sensor Circuit Malfunction | Bad knock sensor, wiring issue, engine knock | Yes — reduced power |
| P0340 | Camshaft Position Sensor Circuit | Bad cam sensor, wiring, timing chain stretch | Yes — may stall |
| P0401 | EGR Insufficient Flow | Clogged EGR valve/passages, bad DPFE sensor | Yes — no immediate risk |
| P0420 | Catalyst System Efficiency Below Threshold (Bank 1) | Bad catalytic converter, O2 sensor, exhaust leak | Yes — check soon |
| P0430 | Catalyst System Efficiency Below Threshold (Bank 2) | Same as P0420 but for bank 2 | Yes — check soon |
| P0440 | EVAP System Malfunction | Loose gas cap (most common), EVAP leak, bad purge valve | Yes — tighten gas cap first |
| P0442 | EVAP System Small Leak | Loose gas cap, small EVAP hose leak | Yes — tighten gas cap |
| P0455 | EVAP System Large Leak | Missing/loose gas cap, broken EVAP hose | Yes — check gas cap |
| P0505 | Idle Control System Malfunction | Dirty throttle body, vacuum leak, bad IAC valve | Yes — may stall |
| P0700 | Transmission Control System Malfunction | Transmission fault — needs deeper scan for specific TCM codes | Yes — check soon |
| P0715 | Input/Turbine Speed Sensor Circuit | Bad speed sensor, wiring, dirty transmission fluid | Yes — may shift poorly |
| P0740 | Torque Converter Clutch Circuit | Bad TCC solenoid, old fluid, worn torque converter | Yes — check fluid level |
| P0741 | Torque Converter Clutch Stuck Off | Bad TCC solenoid, transmission fluid issue | Yes — higher RPM on highway |
| P1128 | Fuel System Too Lean (Manufacturer-Specific) | See P0171 — same cause, manufacturer-specific code | Yes |
Body Codes (B0xxx)
| Code | Meaning | System |
|---|---|---|
| B0001 | Driver Frontal Stage 1 Deployment Control | Airbag — see professional |
| B0028 | Right Side Airbag Deployment Control | Airbag — see professional |
| B0090 | Left Front Restraints Sensor | Airbag sensor fault |
| B1200 | Climate Control Button Stuck | HVAC panel fault |
| B1242 | Wireless Accessory Charging Module | Wireless charger fault |
| B1400 | Driver Door Ajar Circuit | Door switch or wiring fault |
Network Codes (U0xxx)
| Code | Meaning | Common Cause |
|---|---|---|
| U0100 | Lost Communication with ECM/PCM | Bad ECM, CAN bus wiring, low battery voltage |
| U0101 | Lost Communication with TCM | Bad TCM, wiring, transmission range sensor |
| U0121 | Lost Communication with ABS Module | ABS module fault, wheel speed sensor |
| U0140 | Lost Communication with BCM | BCM fault, CAN network issue |
What to Do When a Code Appears
- Don’t panic. Most codes are emissions-related and don’t require immediate stopping.
- Write down the code. You’ll need the exact code (P0420, not “something about the catalytic converter”).
- Check the gas cap. EVAP codes (P0440-P0457) are often just a loose gas cap — tighten it and drive for a day.
- Look up your code here or with a scanner app. OBD2 scanner apps like Torque Pro or OBD Fusion give you a plain-English description.
- See our detailed diagnosis guides for step-by-step testing procedures for common problems.
Last updated: June 2026. This table covers the most common generic and manufacturer codes. For complete code lists, see our OBD2 Scanner Guide.