P0411 Generic (SAE)

Secondary Air Injection System Incorrect Flow Detected

Updated 2026-10-02 · Auxiliary emission controls

Severity

Low

Typical repair cost

40 – 800 EUR

System

Powertrain

What it means

P0411 means the ECM commanded secondary air injection and measured a flow that was either too low or present at the wrong time. Detection is usually via the upstream oxygen sensor response, or on newer systems via a dedicated pressure sensor in the secondary air line. Compared with P0410, this code more specifically indicates a flow restriction or leak: the pump may be running but the air is not reaching the exhaust, or the check valve is leaking exhaust back.

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Symptoms

Check engine light after cold starts
Pump runs but hissing noise from a leaking hose or fitting under the bonnet
Rough idle during warm-up, occasionally stalling in the first 30 seconds
Burnt rubber smell from a hose melted on the manifold
Emissions test failure on cold-start HC and CO

Common causes

Combination valve stuck closed or opening only partially (carbon, corrosion)
Leaking or disconnected secondary air hose between pump and valve
Carbon-blocked air passages in the cylinder head or exhaust manifold
Weak pump delivering reduced flow (worn brushes, partially seized)
Vacuum supply to the combination valve leaking or solenoid sluggish
Leaking check valve letting exhaust pulse back into the line and confusing the pressure sensor

How to diagnose

Perform a cold start and watch live data: upstream O2 voltage should fall below 0.2 V (or lambda rise above 1.1) within seconds of the pump starting. Partial response indicates restricted flow.
Inspect all secondary air hoses and clamps from the pump to the combination valve for splits, melting and loose fittings. Pressurise the line with 0.3-0.5 bar of regulated shop air and listen for leaks.
Test the combination valve: apply 400 mbar vacuum to the control port and blow through; it must pass air freely toward the manifold. Resistance to flow or no opening means replace the valve.
Measure pump output: disconnect the outlet hose and use a pressure gauge or simply feel; a good pump produces strong, steady flow with 0.1-0.2 bar back-pressure. A weak pump drawing under 10 A or over 40 A is failing.
Check manifold passages: with the valve removed, blow compressed air into each port; severe restriction means carbon blockage and the manifold must be cleaned.
On systems with a secondary air pressure sensor (e.g. BMW, Mercedes), check its reading at rest matches atmospheric (about 1000 mbar) and rises 50-150 mbar during pump operation.

Frequently asked questions

Is P0411 the pump or the valve?

More often the valve or the passages. If the pump is audibly running normally and drawing normal current, concentrate on the combination valve, hoses and manifold ports.

Why does the code only appear in winter?

Secondary air runs only on cold starts below roughly 35 C intake temperature. In summer the system may not activate often enough to retest and fail.

Can carbon in the manifold passages really block the air?

Yes. On some engines the small drillings coke up completely after 150,000 km. Cleaning requires manifold removal and 2-4 hours of labour.