DTC reference hub
OBD-II trouble codes
OBD-II diagnostic trouble codes (DTCs) are five-character codes — like P0420 or P0301 — that your car's onboard computer stores when it detects a fault. The first letter identifies the system (P=powertrain, B=body, C=chassis, U=network), the next four digits pinpoint the specific fault. Generic P-codes (P0xxx) are standardized worldwide on every OBD-II car since 1996. STEER reads these codes via Bluetooth and explains each one in plain English, with severity and recommended action.
What the first letter tells you
Every DTC starts with a letter that identifies which system reported the fault. Most codes you'll encounter are P-codes (powertrain).
P
Powertrain — engine, transmission, emissions.
B
Body — airbags, seat belts, climate, lighting.
C
Chassis — brakes, steering, suspension, traction.
U
Network / Communication — modules talking to each other.
Misfire (P0300 family)
When one or more cylinders fail to fire correctly. Severity ranges from "log and investigate" to "stop driving immediately to save the catalyst."
EVAP / fuel vapor (P0440 family)
Codes from the sealed evaporative-emissions system. Almost never affect drivability but always fail emissions inspection until cleared.
Catalyst & oxygen sensor (P0420 / P0171 family)
The fuel-mixture and exhaust catalyst codes. P0420 / P0171 are among the highest-search codes overall — they're emissions failures that point at the catalyst, the O2 sensor, or a vacuum leak.
Intake / airflow (P0100 family)
Codes from the Mass Air Flow, MAP, IAT, and throttle-position sensors. The ECM uses these to calculate fueling — bad data here breaks downstream fuel control.
Charging / starting (P0560 family)
Battery, alternator, and system voltage codes. Most of these clear after a battery service or charging-system repair.
Transmission / drivetrain (P0700 family)
Codes from the transmission control module and related drivetrain sensors. Higher-severity than emissions codes — transmission damage compounds quickly.
Engine cooling & EGR (P0128 / P0401)
Coolant thermostat, cooling fan, EGR system. Codes that point at how the engine manages temperature and exhaust gas recirculation.
Important
DTC codes give clues about what went wrong, but they are not a final diagnosis. The same code can have multiple root causes, and codes can also be triggered by unrelated faults. For serious symptoms — or when the code keeps returning after attempted fixes — a professional inspection is recommended.
Frequently asked questions
What is a DTC?+
DTC stands for Diagnostic Trouble Code. When your car's onboard computer (ECM/PCM) detects something it can't fix on its own, it stores a code identifying which system reported the fault. The code itself is a starting point for diagnosis, not a final answer — many DTCs have several possible root causes.
Are P-codes the same across all car brands?+
Generic P-codes (P0xxx) are standardized by SAE J2012 and mean the same thing on every OBD-II-compliant vehicle (every car sold in the US since 1996). Manufacturer-specific P-codes (P1xxx) are not standardized — Ford P1xxx codes mean different things from Toyota P1xxx codes. Always cross-reference manufacturer P-codes against the vehicle's factory service manual.
Why are P0301 / P0442 / P0102 missing from this list?+
Because they've been consolidated into broader canonical guides. P0301–P0308 live in the Cylinder Misfire by Number guide, P0442 / P0455 / P0456 live in the EVAP Leak Guide, and P0102 / P0103 live in the MAF Sensor Codes guide. The old per-code URLs redirect to these canonicals. This was done to fix programmatic template duplication identified in the 2026 content audit.