Insufficient Flow of Secondary Air Injection System (Bank 1)
What Does It Mean?
This is a generic powertrain code, meaning it covers all makes/models from 1996 onward.
However, specific troubleshooting steps vary by vehicle.
The secondary air injection system, commonly found on Audi, BMW, Porsche, and VW vehicles—and may be found on others—includes an air pump, exhaust manifold, intake check valve, vacuum control switch, electrical input circuit for the vacuum control switch, and numerous vacuum hoses.
This system pumps fresh air into the exhaust system during cold starts to supply oxygen and exhaust gases. This ensures more complete combustion of harmful emissions—hydrocarbons. After about a minute, the system shuts off.
If the air pump and check valve are faulty, they are usually easier to replace; however, most vacuum lines are located under or behind the intake manifold, making them hard to access in most cases. Code P0491 indicates a problem with the system—typically that the secondary air flow is too low on bank 1. Bank #1 is the engine side with cylinder #1. For bank #2, see code P0492.
Other secondary air injection system fault codes include P0410, P0411, P0412, P0413, P0414, P0415, P0416, P0417, P0418, P0419, P041F, P044F, and P0492.
Symptoms
Symptoms of a P0491 engine code may include:
Malfunction Indicator Lamp (MIL) illumination
Potential Causes
Potential causes of a P0491 may include:
Faulty exhaust manifold check valve
A fuse or relay for the secondary air injection pump may be faulty
A faulty air pump
Leaking intake hose
A faulty vacuum control switch
A disconnected vacuum line
A leak in the hoses/piping between the secondary air injection pump and the combi or secondary air injection
A secondary air pressure sensor may be faulty
The combi valve itself is faulty
The secondary air injection port in the cylinder head may be clogged with carbon
The secondary air injection ports in the cylinder head may be obstructed
Diagnostic and Repair Procedures
Most diagnostic tests should be performed when the engine is completely cold. Some follow-up tests may need to wait until the engine cools down completely again—usually a few hours. Some tests may also generate additional error codes. Remember to clear all error codes after finding and fixing the issue.
1. Check the pump. Ensure the engine is completely cold. Remove the pressure hose from the pump or the manifold check valve—whichever is easier to access. Have someone start the engine while you check that the pump is running and pumping air from the end of the hose or output fitting (depending on where you disconnected it). If air is being pumped, proceed to step 4; otherwise, go to step 2.
2. Disconnect the pump’s electrical harness connector. Apply 12 volts to the pump using jumper wires. The terminal with the brown wire is ground, and the other terminal is positive. If the pump runs, proceed to step 3; otherwise, the pump must be replaced.
3. Ensure the engine is cold, connect the multimeter, then have someone start the engine for this step. Test the pump harness connector to ensure it has 12 volts by checking the power between the two terminals of the pump harness plug. If you have 12 volts, repeat the first three steps to ensure no mistakes were made. If you don’t have 12 volts, check the fuses and relay. If you find a bad fuse or relay, replace it, then repeat this step to ensure the harness is powered.
4. Ensure the engine is completely cold. Remove the pressure hose from the check valve. Check if air is coming out of the hose when starting the engine. Do not turn off the engine. After a minute, does the valve close and stop the airflow? If so, the check valve is working correctly. If the valve did not operate, proceed to step 5. If the valve remains open, remove the vacuum control hose and start the engine—the engine does not need to be cold for this test. If the valve remains open, the valve is bad; if it closes, the vacuum control switch is likely the issue.
5. For this step, you’ll need a vacuum pump available at auto parts stores. Start the engine with the vacuum pump attached and holding a check valve nipple. If the valve is open, release the vacuum. If the valve closes, the valve is correct. If the valve does not open at all or does not close, then the valve must be replaced.
6. Let the engine cool completely. Attach the vacuum gauge to the control hose at the check valve. Start the engine. You should have at least 10 to 15 inches of vacuum. Otherwise, you may need to take the vehicle to a shop, as further diagnostic tests require removing some engine components. If no vacuum is present or vacuum readings are low, proceed to step 7.
7. Locate your vehicle’s vacuum control switch. Trace the vacuum line from the check valve to the switch. Look for hardened spots in the hose, cracks, or loose connections. Replace the hose if found, then repeat the test in step 6. If you still have no vacuum, you may need to remove engine parts to access the switch.
8. Remove the input vacuum hose from the control switch. Attach the vacuum gauge to the input hose to test manifold vacuum—with the engine running. If you have no vacuum, replace the vacuum hose and repeat this step.
9. Apply vacuum to the input nipple of the vacuum control switch. The valve should be closed, and the pump should not hold vacuum. Apply 12 volts to both terminals of the control switch using jumper wires. If the switch does not open and release vacuum from the pump, then the switch must be replaced.
Secondary Air Injection Video
