Secondary Air Injection System Incorrect Flow Detected

What it means

The engine computer watches the oxygen sensors while the secondary air system is running; if they do not show the expected rise in exhaust oxygen, it sets this code. It means the airflow is lower or higher than required, usually from a weak pump or blocked valves or passages.

Related codes: P0410P0412P0418P0420

Symptoms4

• Check engine light on.
• Occasional abnormal pump noise at cold start.
• Failed emissions test.
• Driving is usually unaffected.

Possible causes6

• Weak or failed secondary air pump.
• Check valve or switching valve stuck closed.
• Air passages in the exhaust manifold or cylinder head clogged with carbon on some engines.
• Leaking or disconnected air hoses.
• Faulty relay or fuse preventing the pump from running.
• Slow upstream oxygen sensor (less common).

How to diagnose5

1. Command the pump with a scan tool and check for airflow at its outlet.
2. Watch the upstream oxygen sensor; its reading should drop clearly while air is injected.
3. Check that the check valve and switching valve open when commanded.
4. Inspect hoses and air passages for blockage or leaks.
5. Check the fuse, relay and voltage at the pump.

How to fix5

1. Repair leaking or disconnected hoses.
2. Replace a faulty relay or fuse.
3. Replace a stuck check valve or switching valve.
4. Clean clogged air passages or replace a weak pump.
5. Clear the codes and recheck after a cold start.

The cost is low for a hose or relay, and rises when the pump is replaced or air passages inside the engine must be cleaned.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions