AIR System Control A Circuit
What it means
P0418 means the engine computer has detected a fault in the control circuit for the secondary air injection ('A') system. That system briefly pumps fresh air into the exhaust at cold start to cut emissions; a control-circuit fault mainly triggers the warning light and affects emissions, not everyday driveability.
Can I keep driving? Drivable — the secondary air system only runs briefly at cold start, so day-to-day driving is unaffected, but the car will fail an emissions test until repaired.
Real-world scan data
Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.
Common symptoms
What you'll notice
- Check-engine light on
- Usually no noticeable driveability change
- Higher emissions during cold starts
- May fail an emissions/smog test
Common causes
Most likely first
- Faulty secondary air injection relay or control solenoidLikely
- Open or shorted control circuit wiringCommon
- Corroded or loose connector in the relay/control circuitCommon
- Failed engine control module (rare)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and inspect the secondary-air relay/solenoid connector and wiring for damage or corrosion.
- Test the relay or control solenoid and verify the ECM's control signal reaches it.
- Check power and ground on the control circuit.
- Replace the relay or control solenoid if testing confirms it. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0418?+
Can I just clear P0418?+
Related codes
Read & clear P0418 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.