Possible causes

– Intake leak
– Damaged AIR system components
– Faulty AIR Solenoid Valve
– Faulty AIR system relay

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0491 Description

The secondary air system detected that there was insufficient mass flow change during pump switching (on/off)

P0491 Information for specific makes

P0491 FORD
P0491 BMW
P0491 AUDI
P0491 VOLKSWAGEN