P0491: Secondary Air Injection System Insufficient Flow — Bank 1
What to do
Driveable with no symptom you would notice. Will fail an emissions test. If only one bank is affected, the pump is probably not the problem.
What this code means
Not enough injected air is reaching bank 1's exhaust. The bank-specific form of P0411, and the distinction is useful: a fault on one bank only points at that side's check valve or passage rather than at the pump, which serves both.
The carbon-blocked passage into the cylinder head is the usual finding on one-bank faults, and it is the reason a new pump does not fix them.
Likely causes
Start here
- Carbon-blocked secondary air passage · moderate
- Seized secondary air check valve · moderate
Less common
- Faulty secondary air switching valve · moderate
- Split or disconnected air hose · easy
- Failed secondary air injection pump · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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