OBD-II Trouble Codes Explained
An OBD-II code is the standardised label your car's computer stores when it detects a fault. The code names the symptom the computer saw, not the part that needs replacing. Each page below explains what the code means, what you will notice, which causes are actually most likely, and what the repair typically costs.
How to read a code
Every code starts with a letter identifying the system: P for powertrain, B for body, C for chassis and U for network. The digits that follow narrow it to a specific fault. A code beginning P0 is generic and means the same thing on every manufacturer's vehicle.
A stored code is a starting point for diagnosis, never a shopping list. P0420 does not mean “buy a catalytic converter” — it means the computer measured something that a failing converter would cause, along with several cheaper faults.
Emissions
Catalyst System Efficiency Below Threshold (Bank 1)
Yes, in most cases
Catalyst System Efficiency Below Threshold (Bank 2)
Yes, in most cases
Oxygen Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
Yes
Oxygen Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)
Yes
Oxygen Sensor Circuit Low Voltage (Bank 1, Sensor 1)
Yes, but not indefinitely
Exhaust Gas Recirculation Flow Insufficient
Yes, with care
Fuel and air metering
Ignition
Cooling
Evaporative emissions
Valve timing
Transmission
Cannot find your code?
Describe the symptom instead. You do not need a code reader to get a useful answer about what your car is doing.
Describe the problem instead