P0491
PowertrainSecondary Air Injection System Insufficient Flow Detected Bank 1
The secondary air injection pump pushes fresh air into the exhaust manifold for the first minute or two after a cold start, which helps the cat heat up faster and burns off the extra fuel a cold engine dumps in. P0491 means the ECU has worked out that not enough air is getting through on bank 1 during that warm-up burst. It's purely an emissions system, so the car will run, but the light stays on and it'll usually trip again next cold morning.
ⓘ Information only. This page provides general educational information about fault code P0491. 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 P0491 mean?
P0491 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Secondary Air Injection System Insufficient Flow Detected Bank 1.
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, often the only thing you'll actually notice
- • A loud whirring or buzzing from the air pump for the first 30-90 seconds after a cold start, sometimes followed by silence if the pump has packed up
- • Slightly rough or lumpy idle while the engine is still cold, smoothing out once it's warm
- • Mild hesitation or flat-spot in the first few minutes of driving from cold
- • On some cars a hissing noise near the pump or hoses, pointing to a split pipe or duff valve
- • Light won't always come back straight away because the test only runs on a proper cold start
Possible causes
Causes commonly associated with P0491, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Seized or failed air pump, the usual offender. They sit low at the front and draw in road spray, and the motor bearings give up over time
- 2. Stuck or corroded check valve, which lets hot exhaust and moisture back into the pump and slowly kills it. Common on VAG petrols like the 1.6 and 2.0 FSI and older BMW straight-sixes
- 3. Carbon clogging the air injection passages in the head or manifold, so even a healthy pump can't shift enough air
- 4. Split, perished or disconnected hose between the pump and the manifold, an easy one to miss on a quick look
- 5. Faulty combi valve or control solenoid not opening the air path when commanded
- 6. Blown fuse, tired relay or a corroded plug on the pump feed, so the pump never spins in the first place
- 7. Wiring or connector corrosion in the control circuit, which is more likely on cars that have done a few salty winters
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. Cold-start the car first thing and listen by the pump for 30 to 90 seconds. A healthy pump whirrs loudly. Dead silence usually means no power or a seized motor, so that tells you which way to head next
- 2. Check the pump fuse and relay, then confirm the pump actually gets 12V during that cold-start window. No feed points at electrical, not mechanical
- 3. Pull the air hose at the manifold end and check for airflow while the pump runs. No air with the pump spinning means a blocked passage, stuck check valve or a split pipe upstream
- 4. Inspect the check valve and combi valve. A check valve that's wet, rusty or won't seal is a classic cause and often cheaper than the pump
- 5. Read live data and freeze frame, and note any P0410 or P0411 family codes sitting alongside it, which narrows things down fast
- 6. If the pump runs, gets air through and the valves are good, look at the head passages for carbon, which is a bigger job and usually a workshop call
Common questions about P0491
What am I likely looking at to get this sorted? +
A check valve or hose is the cheap end, often £40-£120 fitted at an independent. A new air pump is the big one. Aftermarket pumps run roughly £80-£250 and OE can be £300 plus, so figure £200-£500 fitted depending on how buried it is. If carbon has clogged the head passages you're into serious labour and a main dealer could push that towards four figures. An independent will normally come in well under dealer rates for the same job.
How do I work out which part has actually gone on my car? +
Listen on a cold start. If the pump is silent it's almost always electrical (check fuse and relay) or a seized motor. If you can hear it spinning but the code still comes back, the air isn't reaching the exhaust, so pull the hose at the manifold and feel for flow. No flow with a running pump means a stuck check valve, a split pipe or carbon in the passages. That two-minute listen-and-feel test points you at the right component before you spend anything.
Can I clean or fix the air pump myself instead of replacing it? +
You can sometimes free off a seized check valve and clear a blocked hose at home, and that fixes a fair few of these. The pump itself isn't really a clean-and-revive part once the bearings have gone, so if it's silent or screeching you're buying a new one. Carbon in the head passages can be cleaned but it's awkward, often needs the manifold off, and most people hand that to a garage. Spend your effort on the valve and hoses first, they're cheap and a real share of P0491s start there.
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 →