This has a distant ring to it but can't find it on the common list of codes, and don't remember.
Does this one refer to the secondary air system "leak", usually the bad air check valve?
Did the WD 40 trick a year ago which worked well, maybe it's back?
Comments please....thanks!

 

I found this in one of Bill Dearborn's posts: Thanks to Bill!

DTC P0491 or P0492
Circuit Description
The secondary air injection (AIR) pump system is designed to lower exhaust emission levels on a start up. The AIR pump is timed to remain ON for approximately 1 minute or until Closed Loop is achieved.

The powertrain control module (PCM) commands the AIR pump relay ON by supplying a ground to the AIR pump relay control circuit. This action energizes the AIR pump, forcing fresh air into the exhaust stream. The PCM also commands the AIR solenoid ON which applies engine vacuum to the AIR shut-off valve. With vacuum applied to the AIR shut-off valve, airflow from the AIR pump flows through the pipes/hoses to the exhaust check valves, then enters into the exhaust stream. The air, oxygen, that is introduced into the exhaust system accelerates catalyst operation. When inactive, the check exhaust valves and the shut-off valve prevent air flow in either direction.

The PCM detects a system air flow condition by monitoring the heated oxygen sensors (HO2S) and Short Term Fuel Trim during normal Open Loop AIR system operation. This is a passive test. If the passive test indicates a pass, the PCM takes no further action. If the passive test fails or is inconclusive, the diagnostic will proceed with an intrusive or active test. The PCM will command the AIR system ON during Closed Loop operation under normal operating conditions. The active test will pass or fail based on the response from the HO2S. A lean HO2S response indicates that the AIR system is functioning normally. An increasing Short Term Fuel Trim value also indicates a normal functioning system. The AIR diagnostic consists of the passive test and the active tests. The AIR diagnostic requires failure of the passive and active tests on 2 consecutive key cycles to illuminate the malfunction indicator lamp (MIL) and stores a DTC. If the PCM detects that the HO2S and Short Term Fuel Trim did not respond as expected on one of the engine banks, DTC P0491 for bank 1 or DTC P0492 for bank 2 sets.

Conditions for Running the DTC
DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0120, P0125, P0131, P0132, P0133, P0134, P0135, P0136, P0137, P0138, P0140, P0151, P0154, P0155, P0156, P0157, P0158, P0160, P0161, P0171, P0172, P0174, P0175, P0200, P0220, P0300, P0335, P0336, P0351, P0352, P0353, P0354, P0355, P0356, P0357, P0358, P0442, P0443, P0446, P0449, P0455, P0496, P1133, P1134, P1153, P1154, P1258, P2135 are not set.
The mass air flow (MAF) is less than 23 g/s.
The air/fuel ratio is 14.7:1.
The engine load is less than 40 percent.
The ignition voltage is more than 11.7 volts.
The engine speed is more than 850 RPM.
The engine coolant temperature (ECT) is between -10 and +110°C (14-230°F) .
The intake air temperature (IAT) is between -10 and +100°C (14-212°F) .
The fuel system is operating in fuel trim cells 1, 2, 4, 5, or 6.
The engine is not operating in the following Modes:
Power Enrichment
Decel Fuel Shut-off
Catalyst Over-Temperature
Conditions for Setting the DTC
The HO2S voltage does not decrease to less than 222 mV for 1.3 seconds. AND
The short-term fuel trim does not change more than a predetermined value.
Action Taken When the DTC Sets
The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.
Conditions for Clearing the MIL/DTC
The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
Clear the MIL and the DTC with a scan tool.
Conditions for Setting the DTC
The HO2S voltage does not decrease to less than 222 mV for 1.3 seconds. OR
The short-term fuel trim does not change more than a predetermined value.
Diagnostic Aids
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.
For an intermittent condition, refer to .
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.
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

 

OK, thanks, that's it all right! I know it sounded familiar. Hope the WD 40 works for another year, the problem check valve is behind the driver side engine head. As far as codes go, at least this one is relatively minor. Of course it drives everyone nuts because it dosn't appear on most lists.