OBD-II Trouble Codes: Meanings and Severity
A trouble code tells you which system the engine computer is unhappy with, not which part to buy. Pick your code below to see what it means, whether it is safe to keep driving and what to check first.
Emissions & Catalyst
| Code | Meaning | Severity |
|---|---|---|
| P0420 | Catalyst System Efficiency Below Threshold (Bank 1) | Safe to Drive |
| P0430 | Catalyst System Efficiency Below Threshold (Bank 2) | Safe to Drive |
| P0442 | Evaporative Emission System Leak Detected (Small Leak) | Safe to Drive |
| P0455 | Evaporative Emission System Leak Detected (Gross Leak) | Fix Soon |
| P0456 | Evaporative Emission System Leak Detected (Very Small Leak) | Safe to Drive |
Fuel & Air
Ignition & Misfire
Cooling
| Code | Meaning | Severity |
|---|---|---|
| P0128 | Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) | Safe to Drive |
Transmission
| Code | Meaning | Severity |
|---|---|---|
| P0700 | Transmission Control System (MIL Request) | Fix Soon |
Codes by Make
Some codes have causes that are specific to one manufacturer, backed by a technical service bulletin or a known design issue.
How to Read an OBD-II Code
| Position | Example (P0420) | What it tells you |
|---|---|---|
| 1st letter | P | System family: P = powertrain (engine, transmission), B = body, C = chassis, U = network communication. |
| 2nd character | 0 | 0 = generic code with the same meaning on every make; 1 = manufacturer-specific code. |
| 3rd character | 4 | Subsystem: for example 1 and 2 = fuel and air, 3 = ignition and misfires, 4 = emissions controls, 7 = transmission. |
| Last two | 20 | The specific fault within that subsystem. |