P0418
AIR System Control A Circuit
Secondary air injection (emissions)
Generic OBD-II
Medium
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.
Medium
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.
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?+
Yes — the secondary air system only assists emissions at cold start, so performance is unaffected. It should still be fixed to pass emissions testing.
Can I just clear P0418?+
You can, but it returns until the control circuit or relay is repaired.
Related codes
Often seen with P0418
Read & clear P0418 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX