P0418–P0419 Secondary Air Injection System Relay A circuit Malfunction
The ECU sees a problem in this circuit but cannot narrow it to a specific short or open. Start at the connector and the wiring before condemning the part.
Can you drive it? This is an emissions/EVAP fault. It will not hurt the engine and the car drives normally, but it will fail an emissions test and keeps the light on.
Which code is which
Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.
| Code | Definition | Severity |
|---|---|---|
P0418 | Secondary Air Injection System Relay A circuit Malfunction | Not urgent |
P0419 | Secondary Air Injection System Relay B Circuit Malfunction | Not urgent |
The definition
P0418: Secondary Air Injection System Relay A circuit Malfunction.
P0418 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.
What to check first
Cheapest and fastest checks first:
- Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
- Check the connector at the part for corrosion, and that it is fully seated.
- Clear the code and see whether it returns, and under what conditions.
What actually causes this on a VAG
Many VAG petrol engines carry a secondary air injection system: an electric air pump that runs for up to a minute or two after a cold start, blowing fresh air into the exhaust so the catalytic converter heats up and starts working sooner. It is purely a warm-up emissions device and has no effect on how the car drives once warm, so this is not a safety fault. VAG's own wording for P0418 is "Secondary Air Injection System Relay A Circuit" - it concerns the relay that switches the pump on, not the pump's plumbing, so it is an electrical fault in the switching circuit.
Most likely causes, in order
- The relay itself. It switches a heavy current to an electric motor, so its contacts burn or its coil goes open circuit in normal service. It is a cheap part. Location varies by model - usually the engine-bay relay and fuse box, though on some cars it sits on the relay panel under the dash. If an identical relay, same part number rather than just the same shape, is used elsewhere in the box, swapping the two is a quick and free test.
- The fuse or its holder. The pump is fed through a high-current fuse, usually in the engine-bay box. Check the fuse itself and, just as importantly, whether it is a tight fit and the holder is not discoloured. A fuse that keeps blowing is telling you the pump is dragging too much current - never fit a larger one to stop it blowing.
- Wiring and connectors to the pump and relay. Depending on the model the pump sits in the engine bay or low down behind the bumper or wheel arch liner, where it gets road salt and spray, so its connector corrodes and the loom suffers. Inspect the plug at the pump and at the relay base for green or burnt terminals.
- A seized or water-damaged pump killing the relay. A pump that has ingested water or seized draws far more current than the relay is designed for and destroys it. Listen on a genuine cold start: a healthy pump makes a clear whirring noise for anything from around half a minute to a couple of minutes depending on temperature. Silence, or a rough grinding, points here.
- The relay control wire or the ECU output driver. The ECU normally switches the relay by supplying its earth through a thin control wire, and a break in that wire produces the same code. Genuinely uncommon, and the ECU itself more so - check these last, once the relay, fuse, loom and pump have all been proved good.
Worth knowing: This system only runs on a real cold start, so after any repair you must clear the code, leave the car standing overnight and start it cold. A short test drive proves nothing at all. If the pump has drowned, find out why before fitting another: a leaking combination valve letting hot exhaust moisture back up the pipe, or an air inlet drawing from a wet area, will kill the replacement the same way. And although the car drives perfectly with this fault, in the UK an illuminated engine management light is a major defect and an MOT failure, so it does need fixing.
Reading the code is the easy part
A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.
VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.
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Common questions
What does P0418 mean?
P0418 means Secondary Air Injection System Relay A circuit Malfunction. The ECU sees a problem in this circuit but cannot narrow it to a specific short or open. Start at the connector and the wiring before condemning the part.
Can I drive with P0418?
This is an emissions/EVAP fault. It will not hurt the engine and the car drives normally, but it will fail an emissions test and keeps the light on.