Secondary Air Injection System Incorrect Flow Detected

On many gas vehicles there's a smog/air pump that runs for the first 60–90 seconds after a cold start. It pushes fresh air into the exhaust ahead of the cats to help them light off faster and burn off the rich cold-start mixture. The PCM watches the upstream O2 sensors during this — they should swing lean when the pump kicks in. If they don't swing lean (no air getting through) or swing too lean (way more air than expected) the code sets. Usually it's a seized pump from moisture, a stuck check valve, or rotted hoses.

During the cold-start air injection event, upstream O2 voltage doesn't shift toward lean the expected amount, or fuel trim correction during the event is outside calibration.

Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.

1. 1Cold-start the vehicle and listen. Pump should run audibly for the first minute. Silence = pump, relay, or fuse.
2. 2Check the fuse and relay first. Free check, often the answer.
3. 3If pump runs, feel each hose for airflow. No flow downstream = check valve stuck or passages plugged.
4. 4Scan-tool command the pump on (where supported) and watch O2 voltages drop. Confirms the air is making it into the exhaust.
5. 5On VW/Audi 2.0T and similar, the head passages carbon up and physically block the air. Pump and valves test fine — but air still can't reach the exhaust. Pull the combi valve and look.
6. 6If replacing the pump, also replace the check valve. Failed pumps usually take the check valve with them because exhaust pulses backfeed into the dead pump and rust it solid.

• Failed / seized secondary air pump (#1 cause — water intrusion)
• Stuck or corroded combination valve / check valve
• Cracked or disconnected air injection hoses
• Failed pump relay or blown fuse
• Carboned-up air injection passages in the head (VW/Audi especially)

Pump $200–$800. Combination valve $100–$400. Hoses $30–$150. Decarbon head passages $400–$1,200 labor.