P0412
PowertrainSecondary Air Injection System Control A Circuit
The ECU has spotted an electrical problem in the circuit that drives the secondary air injection switching valve. That valve only does anything for the first minute or two after a cold start, when the system pumps fresh air into the exhaust to help the cat heat up and burn off the extra fuel from cold running. For you the day-to-day driving is fine, but it's an emissions fault, so the light stays on and you've got an MOT problem waiting if you don't sort it.
ⓘ Information only. This page provides general educational information about fault code P0412. 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 P0412 mean?
P0412 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Secondary Air Injection System Control A 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, usually the first and often only thing you notice
- • Slightly lumpy or hesitant idle from cold, smoothing out once the engine warms through
- • An odd buzzing or whirring from the air pump area on a cold morning, or the pump staying silent when it should be running
- • Marginally worse fuel economy, the sort you'd only spot over a full tank
- • Higher emissions at the tailpipe, which only really shows up on a proper test
Possible causes
Causes commonly associated with P0412, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed switching valve, either the solenoid windings have gone open-circuit or the valve has stuck. This is the usual offender, especially on cars that do lots of short hops and never get warm
- 2. Corroded or water-damaged connectors at the valve. The whole assembly often sits low and exposed, so road spray and salt get into the plug
- 3. Chafed or broken wiring between the ECU and the valve, common where the loom rubs on a bracket or the heat shield
- 4. Blown fuse or a dead relay feeding the secondary air circuit, worth ten seconds with a test light before you blame anything pricey
- 5. Water sitting inside the air pump or hoses, which corrodes the internals and can take the valve circuit with it. Common on the older M54 and N42 BMW petrols, also seen on VW and Audi 1.6 and 2.0 petrols
- 6. Carbon and gunk blocking the air passages so the valve can't seat properly, which on some cars shows as an electrical fault because the ECU sees no movement
- 7. ECU driver fault, rare, but possible if everything downstream checks out clean
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 full code list and freeze frame data. If you've got P0410, P0411 or a flow code sat next to P0412 you're looking at the air pump or plumbing, not just the wiring
- 2. Get under the car and eyeball the valve, its connector and the loom. Pull the plug and check for green corrosion, damp, or pins that have backed out. This finds the fault on a fair number of cars before you touch a meter
- 3. Back-probe the connector with a multimeter and have someone fire it up cold. You should see the ECU command the valve briefly in the first 30 to 90 seconds. No command means look upstream at fuse, relay and ECU
- 4. With the plug off, measure the resistance across the valve solenoid and compare to the workshop figure, typically a low number in the single-digit to low tens of ohms. Open circuit or way out of spec condemns the valve
- 5. Check the feed and ground are actually getting to the connector when commanded. A good earth is the thing people skip, and it causes plenty of these
- 6. Fix what you found, clear the code, then do a few proper cold starts. The system only tests itself from cold, so a quick warm drive round the block won't tell you anything
Common questions about P0412
What am I likely to pay to get this fixed? +
If it's a corroded connector or a chafed wire, an independent garage might sort it for £50 to £100 including the diagnostic time. A replacement switching valve is usually the bigger cost, parts run roughly £80 to £250 depending on the car, with another hour or so of labour on top, so call it £150 to £400 fitted at a good independent. A main dealer will charge more for the same job, often £400 plus, and if the whole air pump assembly has corroded internally on a BMW or Audi you can be looking at the high hundreds. Get a proper diagnosis before anyone orders parts.
How do I work out which of these it actually is on my car? +
Start with the connector and wiring because it's free to check and it's a common cause on anything that's seen a few British winters. If the plug is clean and the wiring's sound, measure the solenoid resistance. An open or out-of-spec reading points straight at the valve. If the valve checks out fine electrically but you've also got a flow or pump code stored, the problem is the air pump or blocked passages rather than the switching circuit. The freeze frame data telling you it only fails from cold is your clue that everything's pointing at the secondary air system and not a random electrical gremlin.
Can I clean it or fit a valve myself? +
Yes, both are within reach if you're handy. Cleaning a crusty connector and treating it with dielectric grease costs almost nothing and fixes a decent share of these. Swapping the valve is usually a couple of bolts and a vacuum or air line, though on some engines it's buried and you'll be working blind. What you can't really DIY is tracing an intermittent wire or proving a dead ECU output, that wants a wiring diagram and a meter you trust. If you've cleaned the plug and the code comes straight back, get it on a scanner that shows live valve commands.
If I just clear the code, does it stay gone? +
If the fault's still there it'll come back, but not always straight away. The system only checks the valve from a proper cold start, so the light might stay off for a day or two of short trips before it relogs on the next cold morning. Clearing it without fixing it just hides the problem and risks masking a second fault that sets later. Fix the cause, clear it, then run a few cold starts to make sure it's properly gone before you trust it for an MOT.
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 →