P0491
UniversalSecondary Air Injection System (Bank 2)
The secondary air injection system sends air into the exhaust during certain operating conditions to help reduce emissions. This code indicates a detected concern with that system for Bank 2. The exact monitor and failure condition depend on the vehicle; the code alone does not prove that a particular part has failed. The check-engine light may be the only noticeable effect.
⚠️ Before You Begin (Safety First)
This issue is generally emissions-related and does not usually require stopping immediately. Avoid touching or testing hot exhaust components or moving parts. Stop driving and seek help if the engine runs abnormally, loses power, overheats, or exhaust fumes enter the cabin.
Common Symptoms
- The check-engine light may illuminate, sometimes with no noticeable change in how the vehicle drives.
- An emissions test may be affected because the secondary air system helps reduce exhaust emissions.
- Some vehicles may have rougher operation during startup or other operating conditions when the system is expected to work, though this code does not establish that symptom.
Common Causes
- A pump or control valve may not operate as expected.
- A hose, connection, or air passage may be loose, damaged, restricted, or leaking.
- Wiring, connectors, a fuse, or a control circuit may have an electrical fault.
- A monitoring input or an issue elsewhere in the system may lead the vehicle computer to detect abnormal operation.
Diagnostic Steps
- Record the code and any available freeze-frame information before clearing anything; this can help show when the concern was detected.
- With the engine off and cool, visually inspect accessible secondary-air hoses and connections for looseness, cracking, or obvious damage.
- Check accessible electrical connectors for damage or corrosion without forcing them or probing terminals.
- If checking a fuse, use the vehicle owner's manual to identify the correct one and replace it only with the specified type and rating; do not bypass it.
- If the code returns or checks require access to hot, moving, or hard-to-reach components, have a qualified technician follow vehicle-specific diagnostic procedures.
When to Call a Professional
Confirm the code and review available freeze-frame and scan data before replacing parts. Use vehicle-specific service information to check the Bank 2 system's operation and exhaust response. Inspect the applicable pump, valves, hoses, passages, wiring, and connections; test electrical supply and system operation using procedures for the exact model, year, and engine.
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Commonly Asked Questions
Does P0491 prove the air pump has failed?
No. It identifies a detected concern with the secondary air injection system for Bank 2, not a confirmed failed part. Hoses, valves, passages, wiring, or monitoring inputs may also need investigation.
What does Bank 2 mean?
Bank 2 is the engine bank designated by the vehicle manufacturer. Its location depends on the engine layout, so use model-, year-, and engine-specific information to identify it.
Can I drive with P0491?
If the vehicle otherwise runs normally, this code is generally emissions-related and is not usually an immediate reason to stop. Arrange diagnosis, and stop if the vehicle develops serious drivability problems, overheats, or exhaust fumes enter the cabin.
Why might there be no noticeable symptoms?
The secondary air system primarily supports emissions control under particular operating conditions. A fault may turn on the check-engine light without causing an obvious change in everyday driving.
Browse Available Code Guides
Previous and next available guides in database record order, across makes. Adjacent entries may share a code number and do not necessarily describe related faults.