Symptoms of P0428
- Check-engine light, often with no obvious change in how the car drives.
- A failed emissions test or incomplete readiness monitors.
- Sometimes a faint fuel-vapour smell (on EVAP codes).
Common causes of P0428
- A worn catalytic converter or an oxygen/air-fuel sensor reading out of range.
- EVAP leaks — a loose or failed gas cap, cracked hose, or purge/vent valve.
- Exhaust leaks, fuel-trim problems, or a fault in a related monitor.
Useful checks before replacing parts
- Review temperature sensor voltage, reference voltage, ground integrity, freeze-frame load, actual exhaust heat, and other high-input sensor codes.
- Inspect for open grounds, short-to-voltage faults, connector heat damage, harness routing near exhaust, and sensor internal failure.
- Compare cold and hot readings to determine whether the signal is stuck high or reacting to true excessive exhaust temperature.
Related OBD-II codes
These nearby emissions entries can help compare wording and pin down the fault area.