Symptoms of P0418
- 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 P0418
- 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
- Check control A relay command, pump feed voltage, fuse condition, ground path, current draw, and cold-start secondary air monitor data.
- Inspect relays, pump wiring, connectors, water intrusion, seized pump load, check valves, and harness damage near exhaust heat.
- Confirm whether control A can energize the pump or valve path and whether the monitored current matches the commanded state.
Related OBD-II codes
These nearby emissions entries can help compare wording and pin down the fault area.