P0418: Secondary air injection relay A circuit malfunction
The ECU sees a fault in the circuit of the relay that powers the secondary air pump. It is usually the relay itself, a fuse or the wiring.
| Code | P0418 |
|---|---|
| System | Engine and transmission |
| Group | P04xx, Emissions control |
| Type | Generic: it means the same on every make |
| Can you keep driving? | Yes, no rush |
What P0418 means
The secondary air pump draws a lot of current, so the ECU switches it on through a relay. It has detected that the relay control circuit is not responding properly. If the relay sticks on, the pump may run constantly and burn out.
Most common causes
- Faulty or stuck secondary air pump relay
- Blown fuse in the circuit
- Corroded relay or pump connector
- Relay control wiring broken or shorted
- Shorted pump damaging the relay
Symptoms
- Engine warning light on
- Pump does not run from cold
- Pump runs constantly, even when warm
- Failing an emissions test
Can you drive with P0418?
Yes, no rush You can drive. If you hear the pump running all the time, have it seen soon as it may burn out; otherwise there is no rush.
How to check it
Listen to the pump: it should only run for the first minute after a cold start. If it never runs or never stops, swapping the relay for an identical one from the fuse box is a quick test a garage can do.
How to read and clear P0418 with Final OBD
Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0418 with this same explanation and whether it’s confirmed, pending or permanent.
With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.
Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.
Similar codes
- P0409 EGR sensor A circuit malfunction
- P0410 Secondary air injection system malfunction
- P0411 Secondary air injection incorrect flow detected
- P0412 Secondary air switching valve A circuit malfunction
- P0420 Catalytic converter efficiency below threshold (bank 1)
- P0421 Warm up catalyst efficiency below threshold (bank 1)
- P0430 Catalytic converter efficiency below threshold (bank 2)
- P0431 Warm up catalyst efficiency below threshold (bank 2)
- P0400 Exhaust gas recirculation (EGR) flow malfunction
- P0401 Exhaust gas recirculation (EGR) flow insufficient
- P0402 Exhaust gas recirculation (EGR) flow excessive
- P0403 EGR control circuit malfunction
P0418 is a generic code, so it means the same on any make, for example on BMW, Fiat, Ford, Mazda, Peugeot, Volkswagen. See what Final OBD does on your car in makes and models.
What does code P0418 mean?
The secondary air pump draws a lot of current, so the ECU switches it on through a relay. It has detected that the relay control circuit is not responding properly. If the relay sticks on, the pump may run constantly and burn out.
Can you drive with P0418?
You can drive. If you hear the pump running all the time, have it seen soon as it may burn out; otherwise there is no rush.
How do you clear P0418?
With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0418 comes back.
Read it on your car
Final OBD reads the faults of every control unit for free, including each make’s own codes, and with Pro explains each one in plain English. Free clears engine faults at no cost.
Coming soon
Mid to late October 2026
If you want to try it in your car before anyone else, send us your email and we will add you as a tester.
info@finalobd.com