P0411 — Secondary Air Injection System — Incorrect Flow

What P0411 means

On cold startup, the secondary air injection pump pushes fresh air into the exhaust to speed up catalytic converter light-off and reduce cold-start emissions. The ECM checks the downstream oxygen sensor to confirm air flow actually happened — P0411 means it didn't see the expected O2 voltage shift.

Most likely causes (in order)

1. Secondary air pump seized or motor burned out (very common on high-mileage vehicles)
2. Check valve clogged or failed (prevents exhaust backflow into pump — fails closed over time)
3. Air injection hoses collapsed, cracked, or disconnected
4. Combination valve clogged with carbon
5. Pump relay or fuse blown

Symptoms you might notice

• CEL only
• no driveability symptom in most cases
• cold start emissions elevated

What to check first

Listen for the secondary air pump to run for about 30–60 seconds on a cold start (it's a loud whirring near the fender or firewall) — if you hear nothing, the pump, fuse, or relay is the starting point. If you hear it run but P0411 still sets, the check valve or passages are blocked.

Repair cost & difficulty

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