Error code

OBD2 P0491

OBD2P0491
Diagnostic Trouble Code· Standard OBD-II fault code read from the vehicle's onboard diagnostics.

What it means

Secondary Air Injection System Insufficient Flow.

Severity

moderateCan drive, but may experience issues during warm-up and could fail emissions tests.

Summary

P0491 indicates a malfunction in the Secondary Air Injection (SAI) system on bank 1 of the engine. This can mean insufficient air flow or no air injection at all.

Symptoms

  • Mechanical noises from the air pump
  • Hesitation upon acceleration during warm-up cycle
  • Rough idle or unexpected stalls during warm-up

Common Causes

  • Defective check valves allowing water to freeze in the air pump
  • Faulty air pump
  • Damaged wiring and connectors
  • Blown fuse (if fitted)
  • Defective relay controlling the air pump
  • Vacuum leaks affecting check valve operation
  • Leaks in pressure hoses
  • Blocked air passages preventing injected air from reaching the catalytic converter

Diagnosis

  1. Record all fault codes and freeze frame data.
  2. Check the air pump fuse and relay.
  3. Apply direct current to the air pump terminals if it does not start.
  4. Test wiring for ground connectivity, continuity, and resistance.
  5. Replace faulty components as needed.

Fixes

  • Replace defective check valves or air pump

FAQ

Can I drive with P0491?

Yes, but you may experience issues during warm-up and could fail emissions tests.

What causes P0491?

Common causes include faulty check valves, air pump failure, wiring issues, blown fuses, vacuum leaks, and blocked passages.

View source

Not affiliated with OBD2. Answer summarized from the manufacturer's manual linked above; consult that source for the authoritative procedure.

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