P0418AIR System Control A Circuit
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- After a cold start the secondary air (AIR) pump blows fresh air behind the exhaust valves so that unburnt fuel burns off and the catalyst heats up faster; a shut-off valve opens the path and stops exhaust from flowing back. Control A is the drive circuit of pump A – in practice the coil of the pump relay, which the module switches to ground; the old wording was Relay A. GM uses P0418 for an open circuit (200 kΩ or more, 20 of 25 samples, MIL after two trips) and P2257/P2258 for shorts; Saab and Toyota describe the same circuit. If the pump does not run, P0411 may follow. A relay with welded contacts keeps the pump running – Subaru recalled cars because the pump could overheat.
- What you may notice
- Check engine light on
- Pump does not run after a cold start – or runs constantly if the relay is welded
- Secondary air readiness not set (emission test)
- Hot pump or burning smell if it runs constantly
- Likely causes
- LikelyPump relay faulty (coil open, contacts burnt or welded)
- LikelyControl wire or connector to the relay open or corroded
- PossibleFuse blown, often due to a seized pump
- PossiblePump stiff or blocked by water or debris
- RareDriver in the engine module
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €20 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.