P0491
UniversalSecondary Air Injection System Insufficient Flow (Bank 2)
Medium SeverityThe P0491 code indicates that the engine's computer has detected insufficient airflow in the secondary air injection system on Bank 2, which is the side of the engine without cylinder 1. This system is designed to pump fresh air into the exhaust stream during cold starts to help reduce emissions, and a fault can lead to a failed emissions test and illumination of the Check Engine Light.
⚠️ Before You Begin (Safety First)
Always ensure the vehicle is on a level surface and supported by jack stands if lifting it. Disconnect the battery before working on electrical components. Allow the engine to cool completely before inspecting components to avoid burns. Wear appropriate personal protective equipment, including safety glasses and gloves.
Common Symptoms
- Check Engine Light illumination
- Failed emissions inspection
- Rough idle during cold starts (less common)
- Engine may run rich during warm-up (less common)
Common Causes
- Faulty secondary air injection pump (air pump)
- Stuck open or closed secondary air injection check valve (Bank 2)
- Clogged secondary air injection system hoses or passages
- Faulty secondary air injection pump relay
- Blown fuse for the secondary air injection pump
- Wiring issues to the air pump or check valve
- Vacuum leaks affecting the diverter valve (if applicable)
Frequently reported on: Honda CR-V 2002-2006, Toyota Tacoma 2005-2015, Subaru Forester 2006-2009, GM Trucks (various models) 2000-2007
Diagnostic Steps
- Check for a blown fuse associated with the secondary air injection pump and replace if necessary.
- Inspect all vacuum lines connected to the secondary air injection system for cracks, disconnections, or leaks. Repair or replace as needed.
- Locate the secondary air injection pump and its relay. Test the relay for proper function or swap it with a known good relay (e.g., from the horn) to see if the pump activates.
- Visually inspect the secondary air injection pump for any signs of damage, corrosion, or water ingress. Listen for pump operation during a cold start.
- Inspect the secondary air injection check valve(s) on Bank 2 for blockages or signs of failure. Some valves can be tested by attempting to blow air through them (should only flow in one direction).
- If the pump and check valves appear functional and hoses are clear, consider replacing the secondary air injection pump or check valve on Bank 2 if symptoms persist.
👨🔧 When to Call a Professional
If you've checked the basic components like fuses, relays, vacuum lines, and visually inspected the pump and valves without finding a clear fault, or if the problem persists after replacing a suspected part, it's time to consult a professional technician. Diagnosing electrical issues or internal component failures within the air pump or check valves can be complex and may require specialized diagnostic tools.
Commonly Asked Questions
What is the secondary air injection system?
The secondary air injection system pumps fresh air into the exhaust manifold during a cold start. This additional oxygen helps burn off uncombusted fuel, reducing harmful emissions before the catalytic converter reaches operating temperature.
Is it safe to drive with a P0491 code?
While it's generally safe to drive with a P0491 code as it primarily affects emissions, it's best to address it promptly. Prolonged driving with the issue can cause a failed emissions test and potentially damage the catalytic converter over time.
How do I determine if it's the air pump or a check valve?
During a cold start, listen for the secondary air pump to operate. If it runs but the code persists, inspect the check valve(s) on Bank 2 for blockages or proper one-way flow. A failing pump might make excessive noise or no noise at all. A smoke test can help detect leaks.
What is Bank 2?
Bank 2 refers to the cylinder bank on the engine that does NOT contain cylinder #1. For V6, V8, or V10 engines, this distinguishes between the two sides. On many inline engines, there's only one 'bank,' but P0491 specifically points to an issue on the 'Bank 2' side if your vehicle has one.
Can a bad relay cause P0491?
Yes, a faulty secondary air injection pump relay can prevent the pump from receiving power, leading to insufficient airflow and triggering the P0491 code. Always check the relay and its fuse as a first diagnostic step.