OBD-II Code P0491 — What It Means & How to Fix It

What Does This Error Mean?

P0491 means the secondary air injection system on engine bank 1 is not working properly. The secondary air injection system pumps fresh air into the exhaust right after a cold start. This helps burn off excess unburned fuel during the critical warm-up phase, reducing harmful emissions. When bank 1 does not show the expected oxygen increase in the exhaust, the PCM sets this code.

Affected Models

Vehicles 1996 and newer with secondary air injection systems
Very common on BMW, Mercedes-Benz, and other German vehicles
Common on GM vehicles (Chevrolet, GMC, Buick) with V6 and V8 engines
Ford and Chrysler trucks with V8 engines
Any vehicle sold in California or states with strict emissions requirements often have this system

Common Causes

Secondary air injection pump has failed — the pump motor burns out over time, especially in humid climates
Air injection check valve is stuck closed — prevents air from reaching the exhaust
Vacuum-operated switching valve has failed or the vacuum hose to it is cracked
Air injection pump relay or fuse has failed, so the pump never receives power
Blocked air injection hoses or pipes due to rust, debris, or collapsed rubber sections

How to Fix It

Check the air injection pump fuse and relay. The pump relay is usually in the underhood fuse box. A blown fuse or bad relay will prevent the pump from running at all, which is the most common and cheapest fix.

The pump only runs for 30 to 90 seconds right after a cold start. You can verify it is running by listening near the pump location immediately after starting a cold engine.
Locate the air injection check valve on bank 1. It connects the pump output hose to the exhaust manifold. Try to blow through it by mouth — air should pass in one direction only. If it is stuck closed, replace it.

Check valves are cheap ($15 to $40) and often the root cause of P0491. They corrode and stick on high-mileage vehicles.
Inspect all rubber hoses in the bank 1 air injection circuit. Look for cracks, collapses, or disconnections. A cracked hose leaks air before it reaches the exhaust, causing insufficient airflow to trigger the code.

Rubber air injection hoses harden and crack with age. Replace any hose that shows cracks or that compresses and does not spring back.
Check the switching valve (also called a diverter valve) that directs air to bank 1. Apply vacuum to its vacuum port and verify it opens and holds vacuum. A valve that does not hold vacuum will not let air reach the exhaust.

Switching valves cost $30 to $80 and are commonly overlooked. They fail when their rubber diaphragm cracks.
If fuse, relay, hoses, check valve, and switching valve are all good, test the air injection pump directly by applying 12V battery power. The pump should run loudly and push air out. A pump that does not run or runs weakly needs replacement.

Air injection pumps burn out when water gets into them — common in vehicles where the pump is mounted low and exposed to road spray.

When to Call a Professional

Replacing an air injection pump is straightforward but pumps can be expensive ($150 to $500). Check the simpler parts first — relay, fuse, check valve, and hoses — before buying a pump. Emissions-related repairs at a shop typically cost $200 to $700 for this system.

Frequently Asked Questions

What is bank 1 on my engine?

Bank 1 is the side of the engine that contains cylinder number 1. On V6 and V8 engines, the cylinders are split into two banks. For most GM vehicles, bank 1 is the driver's side. For most Ford vehicles, it is the passenger's side. Check your service manual for your specific vehicle.

Can I delete the secondary air injection system?

Legally, no — in most states you cannot remove or disable emissions equipment. Removing it will cause a permanent check engine light and an automatic emissions test failure. In states that do not require emissions testing, some owners disable it, but this is not recommended.

Why does the air injection pump only run at startup?

The catalytic converter needs to reach operating temperature to work properly. During the first 60 to 90 seconds of a cold start, the converter is still cold and cannot process exhaust gases well. Pumping fresh air into the exhaust during this window helps burn off extra fuel and speeds up converter warm-up.