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P codepowertrainMedium severity

P0491Secondary Air Injection System Insufficient Flow (Bank 1)

The secondary air injection system is not delivering enough air into the bank 1 exhaust during cold-start operation.

Definition

The secondary air injection (AIR) system pumps fresh air into the exhaust ports during cold starts to burn off rich-start emissions and light off the catalyst faster; the PCM verifies flow by watching the bank 1 oxygen sensor swing lean while the pump runs. P0491 sets when that expected lean shift does not occur or is too small, indicating insufficient airflow reaching bank 1. The MIL illuminates and the vehicle will typically fail an emissions test, and on many European applications repeated cold-start operation with no air flow can hasten catalyst damage.

Common causes

  • Stuck-closed or carbon-clogged air injection check (combination) valve on bank 1
  • Carbon-blocked air distribution passages in the cylinder head or exhaust manifold
  • Weak or failed secondary air pump
  • Cracked, disconnected, or melted air delivery hose
  • Faulty vacuum control solenoid or leaking vacuum line to the combination valve
  • Blown fuse, faulty relay, or wiring fault reducing pump speed

Common misdiagnoses

  • Bank 1 upstream oxygen sensor
  • Secondary air pump replaced when the check valve or passages are blocked
  • Catalytic converter
  • PCM

Troubleshooting steps

  1. 1.Command the AIR system on

    Use a scan tool to activate the secondary air pump on a cold engine and listen for the pump running and airflow at the bank 1 check valve.

  2. 2.Watch the bank 1 O2 sensor response

    With the pump commanded on, the bank 1 upstream O2 voltage should drop lean (below about 0.2 V); no shift confirms air is not reaching that bank.

  3. 3.Test the check/combination valve

    Remove the bank 1 check valve and verify it opens with vacuum or pump pressure and blocks reverse flow; replace it if it is carbon-fouled or seized.

  4. 4.Inspect hoses and vacuum control

    Check the air delivery hose for melting near the exhaust and verify the control solenoid passes vacuum to the valve when energized.

  5. 5.Verify pump output

    Disconnect the outlet hose and confirm strong airflow from the pump; a pump that spins but moves little air, or one full of water, must be replaced.

  6. 6.Check for blocked passages

    If pump and valve are good, blow low-pressure air through the bank 1 distribution passages and clean carbon blockage in the head or manifold ports.

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