OBD2 Fault Code Library: Every P, B, C and U Code Explained with Causes and Fixes
What Are OBD2 Codes and How to Read Them
OBD2 (On-Board Diagnostics II) fault codes are standardized 5-character identifiers stored by your vehicle's ECU when a sensor reading or system behavior falls outside the acceptable range set by the manufacturer. All vehicles sold in the United States since 1996 are required to be OBD2-compliant, meaning the same generic code (P0300, P0420, P0171) has the same general meaning regardless of make or model. To read your codes, plug an OBD2 scanner into the diagnostic port under the driver's side dash. Free code reads are available at AutoZone, O'Reilly, and Advance Auto Parts. Basic Bluetooth OBD2 scanners for smartphones cost $20 to $50 and are worth owning for any driver.
The Four OBD2 Code Categories
P codes (Powertrain) cover engine and transmission faults and are by far the most common category of codes. B codes (Body) cover interior systems, lighting, seat controls, and body control modules. C codes (Chassis) cover ABS, traction control, electronic stability control, and suspension systems. U codes (Network/Communication) cover failures in the CAN bus communication network between control modules. Each code starts with a letter followed by 4 digits — the second digit distinguishes generic (0) from manufacturer-specific (1, 2, 3) codes. Generic codes have the same definition on all vehicles; manufacturer-specific codes require brand-specific documentation.
Most Common OBD2 Codes and What They Mean
P0300 — Random or multiple cylinder misfire detected — most common code across all vehicles, caused by spark plugs, ignition coils, or injectors. P0420 — Catalytic converter efficiency below threshold Bank 1 — often triggers after 100,000 miles, indicates converter degradation. P0171 — System too lean Bank 1 — vacuum leak, dirty MAF sensor, or fuel pressure fault. P0174 — System too lean Bank 2 — same causes as P0171 but affecting the second cylinder bank on V6 and V8 engines. P0442 — Small EVAP system leak detected — frequently just a loose or degraded gas cap. P0455 — Large EVAP system leak — more serious fuel vapor escape, check gas cap, vapor canister, and hoses. P0128 — Coolant temperature below thermostat regulating temperature — usually a stuck-open thermostat. P0401 — EGR flow insufficient — clogged EGR valve or passages restricting exhaust gas recirculation.
When to Clear Codes and When Not To
You can clear any OBD2 code with a scanner. However, clearing without fixing the underlying cause will result in the code returning after a few drive cycles. More importantly, clearing codes resets the OBD2 readiness monitors — the self-tests the system runs during normal driving to verify emission system function. A vehicle with recently cleared codes and incomplete readiness monitors will fail a state emissions inspection even if the check engine light is not on. Only clear codes after confirming the repair resolved the root cause, and complete several normal drive cycles to allow the readiness monitors to reset before any inspection.