OBD-II Codes Explained: A Complete Guide
Key Takeaway
A complete breakdown of OBD-II diagnostic trouble codes. Learn what P0, C0, B0, and U0 codes mean and how to read them.
What Are OBD-II Codes?
On-Board Diagnostics II (OBD-II) is a standardized system that monitors your vehicle's engine, transmission, and emissions. When the system detects a problem, it stores a Diagnostic Trouble Code (DTC) that identifies the specific issue.
Every vehicle sold in the United States since 1996 is required to have an OBD-II port, usually located under the dashboard near the steering column.
DTC Format Breakdown
| Position | Meaning | Values |
|---|---|---|
| 1st char | System | P = Powertrain, C = Chassis, B = Body, U = Network |
| 2nd char | Type | 0 = Generic (SAE), 1 = Manufacturer-specific |
| 3rd char | Subsystem | Fuel, ignition, emissions, etc. |
| 4th–5th | Fault # | Specific fault within that subsystem |
Example: P0171
This code means the engine is getting too much air and not enough fuel on Bank 1.
20 Most Common OBD-II Codes
| Code | Meaning | Severity |
|---|---|---|
| P0171 | System Too Lean (Bank 1) | Medium |
| P0174 | System Too Lean (Bank 2) | Medium |
| P0300 | Random/Multiple Cylinder Misfire | High |
| P0301 | Cylinder 1 Misfire | High |
| P0420 | Catalyst System Below Threshold (Bank 1) | Medium |
| P0430 | Catalyst System Below Threshold (Bank 2) | Medium |
| P0440 | EVAP System General Malfunction | Low |
| P0442 | EVAP System Small Leak | Low |
| P0455 | EVAP System Large Leak | Low |
| P0128 | Coolant Thermostat Below Temp | Medium |
| P0135 | O2 Sensor Heater Circuit (Bank 1 Sensor 1) | Medium |
| P0401 | EGR Insufficient Flow | Medium |
| P0411 | Secondary Air Injection Incorrect Flow | Low |
| P0500 | Vehicle Speed Sensor | Medium |
| P0505 | Idle Air Control System | Medium |
| P0700 | Transmission Control System | High |
| P0715 | Input/Turbine Speed Sensor | High |
| C0035 | Left Front Wheel Speed Sensor | Medium |
| B0100 | Driver Frontal Stage 1 Deployment Control | High |
| U0100 | Lost Communication with ECM/PCM | High |
Generic vs. Manufacturer-Specific Codes
Generic codes (starting with P0, C0, B0, U0) are standardized across all manufacturers. Manufacturer-specific codes (starting with P1, C1, B1, U1) are unique to each brand and often require specialized knowledge or tools.
Steer's VIN-based data mapping automatically identifies your vehicle and provides accurate interpretation for both generic and manufacturer-specific codes.
What to Do After Reading a Code
1. Don't just clear it. Understand the root cause first. Clearing a code without fixing the problem means it will come back.
2. Check severity. Some codes are informational, others require immediate attention.
3. Look for related codes. Multiple codes often share a single root cause.
4. Use Steer. Our AI Mechanic groups related codes, explains the root cause in plain language, and gives you a clear action plan.
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Get instant plain-English explanations for any engine code on your iPhone. Steer stays connected 24/7 to monitor your car's health.
