Blocked or restricted secondary air passages, pipes or injection ports, often from carbon build-up or moisture; Faulty or sticking secondary air combination valve or shut-off valve (stuck closed or leaking); Weak or failed secondary air pump, or a seized or water-damaged pump; Pump relay, fuse, wiring or connector fault giving reduced power to the pump; Faulty vacuum or electric solenoid that controls the air valve, or a split vacuum hose; Cracked, disconnected or leaking air hose between the pump and the valve or exhaust; Faulty bank 1 oxygen sensor or air flow/pressure sensor giving a false reading, where fitted.

P0491

AIR System Insufficient Flow Bank 1.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
4: AreaAuxiliary emission controls
91: FaultThe specific fault in that area

What it means

P0491 means the secondary air injection (AIR) system on bank 1 is not delivering as much air as the engine control unit expects. The control unit checks this, usually just after a cold start while the system is active, by watching the oxygen sensor response or by comparing a modelled flow against the measured result. If the air flow is too low, or the sensors do not react as predicted, the code is stored and the warning light comes on.

Symptoms

Engine warning light on
Whining, buzzing or no noise at all from the secondary air pump after a cold start
Slightly rough or unstable idle on a cold start
Failed emissions test
Possible hissing or rattling noise from the air pipes or valve area
Usually no change in how the car drives

Possible causes

Blocked or restricted secondary air passages, pipes or injection ports, often from carbon build-up or moisturelikely
Faulty or sticking secondary air combination valve or shut-off valve (stuck closed or leaking)likely
Weak or failed secondary air pump, or a seized or water-damaged pumplikely
Pump relay, fuse, wiring or connector fault giving reduced power to the pumppossible
Faulty vacuum or electric solenoid that controls the air valve, or a split vacuum hosepossible
Cracked, disconnected or leaking air hose between the pump and the valve or exhaustpossible
Faulty bank 1 oxygen sensor or air flow/pressure sensor giving a false reading, where fittedrare

How to diagnose

Read all stored codes and freeze frame data, noting engine temperature and conditions when the fault set, and check for related AIR system or oxygen sensor codes
Check the fuse and relay for the secondary air pump, and inspect the pump wiring and connectors for damage, corrosion or water ingress
Listen for the pump running for a short time after a cold start and check for hissing from leaks
Inspect all air hoses, vacuum lines and pipework for splits, loose joints, blockages or moisture
Use a scan tool to activate the secondary air pump and valve, if supported, and check the pump runs and the valve opens
Check the pump supply voltage and earth while it is commanded on, using the manufacturer's wiring information
Test the air valve and its control solenoid for free movement, sealing and correct operation, following the procedure for that model
Check the injection ports in the cylinder head or exhaust manifold for carbon blockage, and monitor the bank 1 oxygen sensor response during the test

How to fix

Clear blocked air passages or injection ports, or replace the affected pipes
Replace a stuck, leaking or faulty secondary air valve
Replace the secondary air pump if it is weak, seized or damaged
Repair wiring, connector, fuse or relay faults, and fix any water ingress
Replace damaged hoses or vacuum lines and the control solenoid if faulty
Replace the oxygen sensor or flow sensor only if testing proves it is giving false readings

Can I keep driving?

Low severity

The car can normally be driven as usual in the short term as this is an emissions fault, but book it in soon. Leaving it may mean a failed emissions test and, if water or debris is involved, further damage to the pump or valve.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.