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DTC codes / P04xx · Auxiliary emission controls / P0491

P0491Secondary Air Injection System Insufficient Flow (Bank 1)

P0491 means the engine computer commanded the secondary air injection system to add fresh air into the exhaust on bank 1, but the oxygen sensors did not see the expected change in exhaust oxygen content. The computer therefore flags insufficient flow through the bank 1 air injection path.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaAuxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light, often with no other symptoms besides the warning light
  • A brief rough idle or stumble during the first minute or two after a cold start
  • A faint exhaust smell or slightly louder exhaust note during cold-start warm-up
  • Slight hesitation or reduced power when the engine is cold and the air pump is commanded on
  • In some vehicles, a whistle or hiss from the engine bay during cold start as air escapes a cracked hose

Common causes, most likely first

  1. Cracked, disconnected, or melted secondary air injection hoses and tubing
  2. Failed secondary air injection pump that does not deliver enough air volume
  3. Stuck-closed or carbon-fouled secondary air injection combi valve on bank 1
  4. Blown fuse or corroded relay in the secondary air pump circuit
  5. Clogged or carbon-blocked air injection passages inside the bank 1 cylinder head or exhaust manifold
  6. Failed or sticking secondary air injection solenoid that controls the combi valve
  7. Exhaust leak upstream of the oxygen sensor that dilutes the air injection effect

How to diagnose and fix it

  1. Inspect all secondary air injection hoses and tubing for cracks, splits, melting, or disconnection, especially near the exhaust manifold
  2. Check the secondary air pump fuse and relay, then command the pump with a scan tool to confirm it runs and delivers airflow
  3. Test the combi valve for free movement and proper sealing, and verify it opens when commanded
  4. Measure vacuum or electrical signal at the combi valve solenoid to confirm the control side is working
  5. Check for exhaust leaks between the cylinder head and the upstream oxygen sensor on bank 1
  6. Verify air injection passages are clear by introducing air at the combi valve and watching oxygen sensor response
  7. Replace the failed component, clear the code, and run the monitor to confirm the repair

What the flow test does not tell you

  • A pump that runs and sounds normal can still deliver too little air volume because of a worn impeller or internal leak, so airflow must be verified rather than assumed from noise alone.
  • Carbon buildup in the cylinder head air passages is a common cause that is easily missed when the external hoses and pump test good, and it often requires mechanical cleaning rather than parts replacement.
  • An exhaust leak upstream of the oxygen sensor can mimic insufficient air flow by letting outside air into the exhaust, so always inspect the exhaust before condemning the air injection system.
  • The combi valve may hold vacuum and move freely on the bench but still leak exhaust gas back into the air pump, which damages the pump and sets flow codes intermittently.
  • On some engines the secondary air injection system only runs for the first few minutes after a cold start, so the fault may not reproduce on a warm engine and the vehicle may need to sit overnight before testing.

Can I keep driving with P0491?

The vehicle is generally safe to drive with P0491 because the fault affects only cold-start emissions and does not impair normal engine operation. Avoid long-term neglect, since a leaking combi valve can allow exhaust gas into the air pump and turn a cheap hose repair into a pump replacement.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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