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P0491: Secondary air injection system insufficient flow (bank 1)

The system that blows air into the exhaust from cold is not delivering the expected air on bank 1. Usually the air pump, the valve or a hose.

Code P0491 at a glance
CodeP0491
SystemEngine and transmission
GroupP04xx, Emissions control
TypeGeneric: it means the same on every make
Can you keep driving?Yes, no rush

What P0491 means

Some petrol engines have a pump that blows fresh air into the exhaust during a cold start, so the catalytic converter warms up faster and emissions drop. The ECU uses the oxygen sensors to check that this air arrives. Here it finds not enough is reaching bank 1. Bank 1 is the side of the engine with cylinder 1.

Most common causes

  • Secondary air check valve or combination valve clogged with carbon or moisture
  • Air hose loose or split, or a broken vacuum control hose
  • Faulty secondary air pump or water inside it
  • Fuse or relay for the pump at fault
  • Faulty electric or vacuum control valve

Symptoms

  • Engine warning light on
  • Loud noise, or no noise at all, from the pump in the first seconds after a cold start
  • The car usually drives normally once warm

Can you drive with P0491?

Yes, no rush It only works for the first few minutes from cold, so it does not affect driving or the engine. It needs fixing for the inspection, but without hurry.

How to check it

Start from cold and listen near the engine: the secondary air pump usually hums for a minute or two and then stops. If you hear nothing, check the fuse and relay; if it runs but the code sets, look at the valve and hoses.

How to read and clear P0491 with Final OBD

Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0491 with this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

P0491 is a generic code, so it means the same on any make, for example on Audi, Fiat, Kia, Mazda, Opel / Vauxhall, Toyota. See what Final OBD does on your car in makes and models.

What does code P0491 mean?

Some petrol engines have a pump that blows fresh air into the exhaust during a cold start, so the catalytic converter warms up faster and emissions drop. The ECU uses the oxygen sensors to check that this air arrives. Here it finds not enough is reaching bank 1. Bank 1 is the side of the engine with cylinder 1.

Can you drive with P0491?

It only works for the first few minutes from cold, so it does not affect driving or the engine. It needs fixing for the inspection, but without hurry.

How do you clear P0491?

With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0491 comes back.