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P0491






P0491-SECONDARY AIR INJECTION SYSTEM INSUFFICIENT FLOW







Theory of Operation

For 20 seconds following a vehicle cold start, the air injection system delivers a controlled amount of outside air into the exhaust system, upstream of the catalytic converters, in order to help reduce hydrocarbon and carbon monoxide gas emissions.

- When Monitored:
After the diagnostics for air injection system stuck closed (P2441) and stuck off (P2445) have passed. ECT above 0 C (32 F), ambient air temperature within approximately 2 degrees of ECT, actual air flow is less than or equal to model air flow, and Secondary Air Flow diagnostic counter above 150 (approximately 4.5 seconds).

- Set Condition:
Actual air flow is less than model air flow, and the difference between model air flow and actual air flow is greater than 10 kg/h.





Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Pre-Diagnostic Troubleshooting Procedure

1. DTC IS ACTIVE

WARNING: The normal operating temperature of the exhaust system is very high. Never work around or attempt to service any part of the exhaust system until it has cooled. Special care should be taken when working near any part of the exhaust system. The temperature rises to a high level after a short period of engine operating time.

NOTE: Diagnose and repair any Secondary Air system sensor signal or relay control circuit DTCs before proceeding with this procedure.

1. Turn the ignition on.
2. With the scan tool, select View DTCs and review the DTC environmental data for this DTC.

Is the good trip counter greater than 0?

Yes

- Refer to the CHECKING FOR AN INTERMITTENT DTC Diagnostic Procedure. Checking for an Intermittent DTC

No

- Go to 2

2. AIR INJECTION SYSTEM COMPONENT INSPECTION

WARNING: The normal operating temperature of the exhaust system is very high. Never work around or attempt to service any part of the exhaust system until it has cooled. Special care should be taken when working near any part of the exhaust system. The temperature rises to a high level after a short period of engine operating time.

1. Turn the ignition off.
2. Inspect the Air Flow (AF) Sensor, the Secondary Air Pump Assembly, the Secondary Air Pump Valve and the connection tubes between them.
3. Look for damage, obstructions, leaks, or any condition that would obstruct or interfere with air flow between the components.
4. Using the wiring diagram/schematic as a guide, inspect the wiring and electrical connectors at the AF Sensor, Secondary Air Pump Relay, and the Secondary Air Pump Assembly.
5. Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact in the related connectors.

Were any problems found?

Yes

- Repair as necessary.
- Perform the PCM Verification Test. PCM Verification Test

No

- Go to 3

3. SECONDARY AIR PUMP ACTUATION
1. Turn the ignition on.
2. With the scan tool, actuate the Secondary Air Pump Relay control to on.
3. With the scan tool, monitor the AF Sensor value.

CAUTION: Do not allow the Air Pump actuation to run for more than 30 seconds at a time. Pump damage may occur. Allow the pump to cool down for at least 10 minutes between actuations.

Is the AF Sensor value greater than 25 kg/h with the pump actuated on?

Yes

- Refer to the CHECKING FOR AN INTERMITTENT DTC Diagnostic Procedure. Checking for an Intermittent DTC

No

- Go to 4

4. SECONDARY AIR CHECK VALVE





WARNING: The normal operating temperature of the exhaust system is very high. Never work around or attempt to service any part of the exhaust system until it has cooled. Special care should be taken when working near any part of the exhaust system. The temperature rises to a high level after a short period of engine operating time.

1. Turn the ignition off.
2. Disconnect the air pump outlet tube (3) from the Secondary Air Check Valve (1).
3. Turn the ignition on.
4. With the scan tool, actuate the Secondary Air Pump Relay control to on.
5. With the scan tool, monitor the AF Sensor value.

CAUTION: Do not allow the Air Pump actuation to run for more than 30 seconds at a time. Pump damage may occur. Allow the pump to cool down for at least 10 minutes between actuations.

Is the AF Sensor value greater than 25 kg/h with the pump actuated on?

Yes

- Replace the Secondary Air Check Valve.
- Perform the PCM Verification Test. PCM Verification Test

No

- Go to 5

5. (Z901) GROUND CIRCUIT OPEN OR HIGH RESISTANCE




1. Turn the ignition off.
2. Disconnect the Secondary Air Pump connector.
3. Using a 12 volt test light connected to 12 volts, check the (Z901) Ground circuit in the Secondary Air Pump harness connector.

NOTE: The test light should be illuminated and bright. Compare the brightness to that of a direct connection to the battery.

Is the test light illuminated and bright?

Yes

- Go to 6

No

- Repair the (Z901) Ground circuit for an open circuit or high resistance.
- Perform the PCM Verification Test. PCM Verification Test

6. SECONDARY AIR PUMP RELAY OUTPUT




1. Turn the ignition on.
2. With the scan tool, actuate the Secondary Air Pump Relay control to on.
3. Using a 12 volt test light connected to ground, check the (A161) Secondary Air Pump Relay Output circuit in the Secondary Air Pump harness connector.

NOTE: The test light should be illuminated and bright. Compare the brightness to that of a direct connection to the battery.

Is the test light illuminated and bright?

Yes

- Go to 7

No

- Go to 16

7. (F342) FUSED ASD/MAIN RELAY OUTPUT CIRCUIT OPEN OR HIGH RESISTANCE




1. Turn the ignition off.
2. Disconnect the AF Sensor connector.
3. Turn the ignition on.
4. Using a 12 volt test light connected to ground, check the (F342) Fused ASD/Main Relay Output circuit in the AF Sensor harness connector.

NOTE: The test light should be illuminated and bright. Compare the brightness to that of a direct connection to the battery.

Is the test light illuminated and bright?

Yes

- Go to 8

No

- Repair the (F342) Fused ASD/Main Relay Output circuit for an open circuit or high resistance.
- Perform the PCM Verification Test. PCM Verification Test

8. (K157) AF SIGNAL CIRCUIT VOLTAGE




1. Turn the ignition on.
2. Measure the voltage of the (K157) AF Signal circuit in the AF Sensor harness connector.

Is the voltage between 4.8 and 5.1 volts?

Yes

- Go to 9

No

- Go to 12

9. (K957) AF SENSOR GROUND CIRCUIT TEST




1. Turn the ignition off.
2. Using a 12 volt test light connected to 12 volts, check the (K957) AF Sensor Ground circuit in the AF Sensor harness connector.

NOTE: The test light should be illuminated and bright. Compare the brightness to that of a direct connection to the battery.

Is the test light illuminated and bright?

Yes

- Go to 10

No

- Go to 14

10. AF SENSOR WIRING AND CONNECTORS
1. Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the AF Sensor and the Powertrain Control Module (PCM).
2. Look for any chafed, pierced, pinched, or partially broken wires.
3. Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact in the related connectors.
4. Monitor the scan tool data relative to this circuit and wiggle test the wiring and connectors.
5. Look for the data to change or for the DTC to reset during the wiggle test.
6. Search for any Technical Service Bulletins that may apply.

Were any problems found?

Yes

- Repair as necessary.
- Perform the PCM Verification Test. PCM Verification Test

No

- Go to 11

11. AF SENSOR
1. Turn the ignition off.
2. Replace the AF Sensor.
3. Verify that all Secondary Air Pump Assembly wiring, connectors, tubes, and hoses are connected properly.
4. Turn the ignition on.
5. With the scan tool, actuate the Secondary Air Pump Relay control to on.
6. With the scan tool, monitor the AF Sensor value.

CAUTION: Do not allow the Air Pump actuation to run for more than 30 seconds at a time. Pump damage may occur. Allow the pump to cool down for at least 10 minutes between actuations.

Is the AF Sensor value greater than 25 kg/h with the pump actuated on?

Yes

- Test Complete.
- Perform the PCM Verification Test. PCM Verification Test

No

- Go to 19

12. (K157) AF SIGNAL CIRCUIT SHORTED TO VOLTAGE




1. Turn the ignition off.
2. Disconnect the Powertrain Control Module (PCM) connector.
3. Turn the ignition on.
4. Measure the voltage of the (K157) AF Signal circuit in the AF Sensor harness connector.

Is there any voltage present?

Yes

- Repair the (K157) AF Signal circuit for a short to voltage.
- Perform the PCM Verification Test. PCM Verification Test

No

- Go to 13

13. (K157) AF SIGNAL CIRCUIT SHORTED TO GROUND




1. Turn the ignition off.
2. Measure the resistance between ground and the (K157) AF Signal circuit in the AF Sensor harness connector.

Is the resistance above 100 ohms?

Yes

- Go to 14

No

- Repair the (K157) AF Signal circuit for a short to ground.
- Perform the PCM Verification Test. PCM Verification Test

14. (K157) AF SIGNAL CIRCUIT OPEN OR HIGH RESISTANCE





NOTE: When checking circuits at the Powertrain Control Module (PCM), be careful not to damage or distort the connector terminals. Improper measurement technique could result in poor pin to terminal contact.

1. Measure the resistance of the (K157) AF Signal circuit between the AF Sensor harness connector and the PCM.

Is the resistance below 5.0 ohms?

Yes

- Go to 20

No

- Repair the (K157) AF Signal circuit for an open circuit or high resistance.
- Perform the PCM Verification Test. PCM Verification Test

15. (K957) AF SENSOR GROUND CIRCUIT OPEN OR HIGH RESISTANCE





NOTE: When checking circuits at the Powertrain Control Module (PCM), be careful not to damage or distort the connector terminals. Improper measurement technique could result in poor pin to terminal contact.

1. Measure the resistance of the (K957) AF Sensor Ground circuit between the AF Sensor harness connector and the PCM.

Is the resistance below 5.0 ohms?

Yes

- Go to 20

No

- Repair the (K957) AF Sensor Ground for an open circuit or high resistance.
- Perform the PCM Verification Test. PCM Verification Test

16. (A161) SECONDARY AIR PUMP RELAY OUTPUT CIRCUIT OPEN OR HIGH RESISTANCE




1. Turn the ignition off.
2. Remove the Secondary Air Pump Relay.
3. Measure the resistance of the (A161) Secondary Air Pump Relay Output circuit between the Secondary Air Pump Relay connector and the Secondary Air Pump harness connector.

Is the resistance below 5.0 ohms?

Yes

- Go to 17

No

- Repair the (A161) Secondary Air Pump Relay Output circuit for an open circuit or high resistance.
- Perform the PCM Verification Test. PCM Verification Test

17. (A903) FUSED B+ CIRCUIT OPEN OR HIGH RESISTANCE




1. Turn the ignition on.
2. Using a 12 volt test light connected to ground, check the (A903) Fused B+ circuit in the Secondary Air Pump Relay connector.

NOTE: The test light should be illuminated and bright. Compare the brightness to that of a direct connection to the battery.

Is the test light illuminated and bright?

Yes

- Replace the Secondary Air Pump Relay.
- Perform the PCM Verification Test. PCM Verification Test

No

- Repair the (A903) Fused B+ circuit for an open circuit or high resistance.
- Perform the PCM Verification Test. PCM Verification Test

18. SECONDARY AIR PUMP RELAY
1. Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the Secondary Air Pump Relay and the Powertrain Control Module (PCM).
2. Look for any chafed, pierced, pinched, or partially broken wires.
3. Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact in the related connectors.
4. Monitor the scan tool data relative to this circuit and wiggle test the wiring and connectors.
5. Look for the data to change or for the DTC to reset during the wiggle test.
6. Search for any Technical Service Bulletins that may apply.

Were any problems found?

Yes

- Repair as necessary.
- Perform the PCM Verification Test. PCM Verification Test

No

- Replace the Secondary Air Pump Relay.
- Perform the PCM Verification Test. PCM Verification Test

19. SECONDARY AIR PUMP
1. Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the Secondary Air Pump and the Powertrain Control Module (PCM).
2. Look for any chafed, pierced, pinched, or partially broken wires.
3. Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact in the related connectors.

Were any problems found?

Yes

- Repair as necessary.
- Perform the PCM Verification Test. PCM Verification Test

No

- Replace the Secondary Air Pump.
- Perform the PCM Verification Test. PCM Verification Test

20. POWERTRAIN CONTROL MODULE (PCM)
1. Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the Secondary Air Pump Assembly and the Powertrain Control Module (PCM).
2. Look for any chafed, pierced, pinched, or partially broken wires.
3. Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact in the related connectors.
4. Monitor the scan tool data relative to this circuit and wiggle test the wiring and connectors.
5. Look for the data to change or for the DTC to reset during the wiggle test.
6. Search for any Technical Service Bulletins that may apply.

Were any problems found?

Yes

- Repair as necessary.
- Perform the PCM Verification Test. PCM Verification Test

No

- Replace the Powertrain Control Module (PCM).
- Perform the PCM Verification Test. PCM Verification Test

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