Diagnostic Aids

WARNING: This page is about a different car, the 2004 Chevrolet Corvette. However, it is still accessible from the selected car via links, so may be relevant.

For any test that requires probing the PCM or probing a component harness connector, use the connector test adapter kit. Using this kit prevents damage to the harness or component terminals. Refer to in Wiring Systems.
Carbon build-up in the exhaust manifold may restrict the amount of air flow necessary to affect the HO2S voltage. If you suspect this condition, remove the air pipe from the manifold and inspect the passage.
Inspect for the following conditions:
- Excessive exhaust system back pressure
- Moisture, water, or debris ingestion into the AIR pump
- Leaking exhaust check valves will leave traces of exhaust carbon in the AIR system.
- A check valve that flows in both directions causes heat damage to the AIR system components.

DTC P0491 or P0492

Step	Action	Values	Yes	No
Schematic Reference: Engine Controls Schematics Connector End View Reference: Powertrain Control Module (PCM) Connector End Views or Engine Controls Connector End Views
1	Did you perform the Diagnostic System Check-Engine Controls?	-	Go to Step 2	Go to Diagnostic System Check - Engine Controls
2	Start and idle the engine until Closed Loop is achieved. Turn OFF all the accessories. Monitor the HO2S bank 1 sensor 1 and bank 2 sensor 1 voltage parameters for the applicable bank with a scan tool. Command the secondary air injection (AIR) system ON with a scan tool. Observe the heated oxygen sensor (HO2S) voltage as the AIR system is enabled. Does the HO2S voltage decrease to less than the specified value?	222 mV	Go to Diagnostic Aids	Go to Step 3
3	Perform a visual inspection of all pipes/hoses in the AIR system for the following conditions: Loose or missing clamps on the AIR pipes/hoses No kinks, holes, or pinched hoses/pipes Components with evidence of heat damage Did you find and correct a condition?	-	Go to Step 7	Go to Step 4
4	Remove the crossover hose from the applicable check valve. Command the AIR pump system ON with a scan tool. Is air flow present at the hose outlet?	-	Go to Step 6	Go to Step 5
5	Repair the restriction or the leak for the applicable bank in the AIR crossover pipes/hoses between the exhaust check valve and the AIR shut-off valve. Did you complete the repair?	-	Go to Step 7	-
6	CAUTION: Refer to Hot Exhaust System Caution in Cautions and Notices. Repair the restriction/leak or blockage for the applicable bank in one of the following components: The exhaust check valve-Refer to Secondary Air Injection (AIR) Check Valve/Pipe Replacement - Bank 1 or Secondary Air Injection (AIR) Check Valve/Pipe Replacement - Bank 2 The exhaust check valve outlet pipe The exhaust manifold Did you complete the repair?	-	Go to Step 7	-
7	Clear the DTCs with a scan tool. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition?	-	Go to Step 2	Go to Step 8
8	Observe the Capture Info with a scan tool. Are there any DTCs that you have not yet diagnosed?	-	Go to Diagnostic Trouble Code (DTC) List	System OK