Description of DTC code P0491

The injection system is composed of the air pump, exhaust manifold, inlet check valve, and vacuum control switch, besides, the switch has an electrical circuit and there are vacuum hoses. This system sends fresh air into the exhaust on cold starts, thus oxygen and exhaust gases are fed. This ensures complete hydrocarbon combustion. After approximately one minute, the system shuts itself down.

The error code P0491 OBDII is set when there is a failure in this system, this usually happens because the secondary airflow is too low in bank 1.

Symptoms of fault code P0491

  • Engine lighting (Check Engine).
  • Mechanical noises from the air pump due to damage.
  • Problems with acceleration during the warm-up cycle.
  • Sudden stagnation in the heating cycle

Causes of OBD2 P0491

The reasons why the DTC code P0491 OBD2 is configured are:

  • The check valve may be defective allowing water to freeze in the air pump during cold periods.
  • The air pump may be damaged.
  • There may be connectors or cables in poor condition.
  • Fuse is blown.
  • Defective air pump relay.
  • Leakage may occur if the injection system controls air check valves.
  • Pressure hoses may leak.


Possible solutions of the DTC code P0491

To solve the failures caused by the diagnostic code P0491 OBDII, you must do this:

  • Check the air pump, to do this the engine must be completely cold. Disconnect the pressure hose. Then, start the vehicle and check if the air is coming out of the hose.
  • Unplug the connector from the pump harness. Then, by using jumper cables, apply 12 volts to the pump. If the pump does not work, it must be replaced.
  • Connect the multimeter, check that 12 volts are present at the pump harness connector with the engine running. If no such voltage is present, check the fuses and the relay, and replace any fuses that do not meet the requirements for proper operation.
  • Remove the pressure hose from the check valve, start the engine, and check that air is coming out of the hose. Also, make sure the valve is working properly. To do this, the valve must open and close, if it remains open, it is defective and must be replaced.
  • Locate the vacuum control switch and inspect the vacuum line from the check valve to the switch. If there are obstructions or cracks in the hose, replace it.
  • Remove the vacuum inlet line at the switch, then connect a vacuum gauge to the inlet hose and check the vacuum in the manifold while the engine is running. If there is no vacuum, change the line.
  • Change the vacuum switch.

Codes related to P0491

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