TORQUETRELLIS / FIELD GUIDES
Start with the code. Keep the context.
Ten plain-language starting points for common OBD-II trouble codes: what they generally report, what you might notice, and when vehicle-specific help matters.
THE LIBRARY
Choose a code to get oriented
- P0300
Random or multiple cylinder misfire detected
What a general misfire result can mean, and what to record before diagnosis.
Read guide - P0301
Cylinder 1 misfire detected
Understand the monitor result without guessing where cylinder 1 sits on your engine.
Read guide - P0171
System too lean, bank 1
A fuel-and-air monitoring result that needs context from the vehicle and related codes.
Read guide - P0420
Catalyst system efficiency below threshold, bank 1
The catalyst monitor reported an out-of-range result; the code does not prove the converter failed.
Read guide - P0442
Evaporative emissions system leak detected, small leak
A small-leak monitor result for the fuel-vapor system, not a location for the leak.
Read guide - P0455
Evaporative emissions system leak detected, large leak
A large-leak or loss-of-seal monitor result that still needs a vehicle-specific check.
Read guide - P0128
Coolant temperature below thermostat regulating temperature
The warm-up monitor saw a temperature result outside its expected range for that vehicle.
Read guide - P0135
Oxygen sensor heater circuit fault, bank 1 sensor 1
A monitored heater-circuit result; the code alone does not identify a failed sensor.
Read guide - P0401
Exhaust gas recirculation flow insufficient
The EGR monitor measured less flow than expected during a test; it does not name the failed part.
Read guide - P0113
Intake air temperature sensor 1 circuit high input
A sensor-circuit reading is outside the expected range; it does not prove the sensor failed.
Read guide