P0418
PowertrainSecondary Air Injection System Control B Circuit
The ECU has spotted an electrical fault in the wiring or relay that switches on your secondary air injection pump. That pump only runs for the first minute or two after a cold start, blowing fresh air into the exhaust to burn off the extra fuel a cold engine produces and bring the cat up to temperature faster. For most owners this shows up as a warning light and nothing else, because the system has done its job by the time you've left your street. The real headache is that it's a known weak point on a lot of VAG and BMW petrols, and chasing it can mean anything from a £5 fuse to a seized pump.
ⓘ Information only. This page provides general educational information about fault code P0418. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0418 mean?
P0418 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Secondary Air Injection System Control B Circuit.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, frequently the only thing you'll ever notice
- • Car drives completely normally most of the time, since the fault sits in a system that only runs cold
- • A loud buzzing or whirring on cold start that suddenly stops, or doesn't happen at all when it should
- • Occasional rough idle or a brief hesitation in the first minute from cold on some engines
- • Limp mode on a few models if the ECU decides the circuit fault is serious enough
- • Higher emissions on a cold-start test, which mainly bites at MOT time
Possible causes
Causes commonly associated with P0418, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Faulty air injection pump relay, the usual offender and the first thing to suspect. They corrode and stick after a few years of heat cycling under the bonnet
- 2. Corroded or chafed wiring in the relay control circuit, common where the loom runs near the pump or in a wet wheel arch
- 3. Blown fuse feeding the secondary air system, often blown by something else upstream so just swapping it rarely fixes it for long
- 4. Seized or failing air pump pulling far too much current, which then takes out the fuse or trips the relay circuit
- 5. Moisture or condensation getting into the pump through a tired check valve, then corroding the motor windings and raising the current draw
- 6. Failed PCM, which is rare and should be the last thing you blame, not the first
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read the live data and check whether the pump actually commands on during a cold start. If you can hear it run, you're chasing a circuit niggle rather than a dead pump
- 2. Pull the relay and inspect it, then check the fuse. A corroded relay or a blown fuse is the cheap, common find and worth ruling out before you touch a multimeter to anything else
- 3. Back-probe the relay control wire and supply with a meter, confirming the ECU is sending the switch signal and the relay is seeing power and a clean earth
- 4. If the fuse keeps blowing, measure the current draw of the pump. A pump pulling well over spec is on its way out and is usually corroded internally
- 5. Inspect the check valve and the pipework into the pump for water ingress, especially on a car that's seen a lot of short winter journeys
- 6. Wiggle-test the loom around the pump and any connectors while watching live data, since intermittent faults here are almost always a chafed wire or a green connector pin
Common questions about P0418
How long should this job take to sort out? +
Diagnosis is the slow bit, budget an hour at a garage to find out whether it's the relay, the wiring or the pump. A relay or fuse swap is a 15 to 30 minute job once you've located them. Chasing a corroded loom can eat a couple of hours if the damage is hidden under the manifold or tucked behind the pump. Replacing the air pump itself is usually an hour or two depending on how tight the access is, and on some BMW N52 and certain VAG petrols it's a proper knuckle-skinner.
Is a cheap aftermarket relay or pump worth fitting, or do I stick with genuine? +
For the relay, a decent branded aftermarket part is fine and there's no reason to pay dealer money for one. The pump is where you want to be choosier. The bargain-basement pumps off the auction sites are notorious for failing again within a year, often because they let moisture straight back in. Pay for a reputable brand or a good OE-equivalent and you'll get years out of it. Spending £60 on a no-name pump to save £80 is a false economy here.
Can I keep driving while this code is up? +
Yes, for normal use you're fine. The secondary air system only works during the warm-up phase and has no part in how the engine runs once it's up to temperature, so your driving won't change. The catch is if the fault is a pump dragging current and blowing fuses, because a shared fuse can take other things down with it. And if it's tripping you into limp mode on a cold morning, get it looked at sooner rather than later.
Will it cost me the MOT? +
The code on its own isn't a fail, but if the engine warning light is lit when the tester looks at the dash, that's an MOT failure on its own regardless of what's behind it. So you'll need the light off, which means fixing the actual cause rather than just clearing it and hoping. On older cars the emissions test is done with a warm engine and won't usually catch a secondary air fault, but the lit MIL will get you either way.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →