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BMW Secondary Air System Fault Code Diagnosing How To DIY OBD-II

August 28, 2012

Some of the most common “check engine” fault codes for BMWs from 1996 through the mid-2000s involve the secondary air injection system. This system consists of an air pump, exhaust manifold input check valve and a vacuum control switch, along with the associated input and vacuum hoses and electrical input circuit to the vacuum control switch. (See illustration below.)

During cold starts, the system pumps fresh air into the exhaust as it exits the engine (at either the exhaust ports in the cylinder head or the exhaust manifold ports). This feeds oxygen to the exhaust gasses, allowing for a more complete burn of hydrocarbons (harmful emissions). The system turns off after about a minute or less.


Fault codes attributable to this system (such as “Secondary Air Flow Too Low”) are usually caused by a faulty air pump and/or exhaust manifold check valve or clogged air injection ports. However, we have found that other parts of this system (including leaking vacuum lines, faulty vacuum control switch, etc.) can be the initial cause of problems, leading to failure of the pump, check valve, relays and fuses. Follow along as we diagnose a secondary air system on a 2001 330i. Note that most 6-cylinder models 1996 through mid-2000s will be similar. (The same diagnostic procedures can be used on V8s and the M3 01 thru 05, but the locations of the parts will vary from what is described here.)

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NOTE: Many of these diagnostic tests must be done with the engine fully cold. This may require some of the follow-up tests to be performed after the engine has cooled down (a few hours). Also, some of the following tests will generate fault codes. When all testing is complete and faulty parts have been replaced, remember to clear all fault codes.

Diagnostic steps:

1    Is the pump running? With the engine fully cold (such as overnight or after a few hours of sitting), remove the pressure hose from the pump. (You can also remove the opposite end of the hose, from the manifold check valve, if access is easier.) Have a helper start the engine. Is the pump running and pumping air out of the outlet nipple (or the end of the hose that was removed from the valve)?

If no, go to step 2; if yes, go to step 4. Note: Turn engine off as quickly as possible, in order to perform the next stage of testing while engine is still cold.

2    Unplug the pump’s electrical harness connector. (This may require the pump to be unbolted from its mounting bracket.) Using jumper wires, apply 12 volts directly to the pump (the terminal that has the brown wire in the harness plug, is ground; the other terminal is positive). Does the pump run?  If yes, go to step 3. If no, pump is faulty and must be replaced. However, you are not finished with the diagnostics; proceed to step 3.

3    Test for 12 volts at the pump harness connector. With the engine fully cold, use a multimeter or 12-volt test light to check for power between the two terminals in the pump harness plug when the engine is started. (Have a helper start the engine so you can test immediately.)

If yes, and the pump did run in step 2, repeat steps 1 through 3 to verify your findings; you may have made a mistake. If no, check the fuses and relay. See applicable Bentley repair manual for details and locations.

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In this case, they are fuses #2 (30-amp) and #3 (20-amp) in the electronics box fuse pack carrier (under hood, driver’s side), fuse #36 (50-amp) in the main fuse panel (above glove box) and the relay in main relay panel (behind glove box). If you find a faulty fuse or relay, correct the fault and repeat step 3 to verify that you have power at the pump’s harness connector. Repeat step 1 if the pump did run in step 2 (pump should now run). If pump did not run in step 2, we need to replace the pump and go on to diagnosing the check valve circuit. It would also be wise to diagnose the check valve circuit even if the pump now runs properly.

At this point, we have determined whether the pump is faulty AND that we have power through the circuit to the pump. We now must determine if the check valve and the vacuum switch circuit are functioning properly.

4 With engine fully cold, remove the pressure hose from the check valve. As in step 1, start the engine and note if exhaust gas is coming from the valve (where the hose was removed), indicating that the valve is open. In this test, continue to run the engine. Does the valve close within a minute or so?

If valve was initially open, then closed, your diagnostics are complete; the valve and control circuit are functioning properly. If valve did not open at all, go to step 5. If valve stayed open, remove the vacuum control hose from the check valve and start the engine (temperature does not matter). If the valve is still open, it is faulty. If valve is now closed, the vacuum control switch is likely faulty.

5    Test for check valve function. Use a vacuum pump (such as a MityVac hand pump, available at auto parts stores, Wal-Mart, Sears, etc.) to apply vacuum to the check-valve vacuum nipple. Start the engine – is the valve open (exhaust coming through the valve’s air input nipple)? Release the vacuum – does the valve close?

If yes, the valve is OK; go to step 6. If the valve did not open at all, or if the valve does not close, the valve is faulty.

6 Test check valve vacuum control circuit. With engine fully cold, install a vacuum gauge to the vacuum control hose that connects to the check valve (remove hose from valve). Start engine and check for vacuum on the gauge. You should have at least 10″ to 15″ of vacuum.

If no vacuum (or very low) and your testing in step 5 showed the check valve to be OK, the diagnostics will be more difficult and may require removal of various engine and/or engine compartment parts. Shut engine off right away, to try to keep it in “cold-start” mode. Before continuing, make sure that the fuses and relay mentioned in step 3 are fully functional, then repeat step 6. If still no vacuum, go on to step 7.

7 Locate the vacuum control switch. On this ‘01 330i, it is located under the rear of the intake manifold and is rather difficult to access. (Access is easier on many other models.) You must remove the microfilter housing, valve cover trim cover and fuel rail trim cover. Follow the vacuum line from the check valve, along the side of the cylinder head, across the rear of the cylinder head, down under the rear of the intake manifold and, ultimately, to the control switch.

As you are tracing the line, be aware of any hardened rubber hose sections and/or cracks or loose connections (creating vacuum leaks). Replace any suspect vacuum hose sections and redo the test in step 6. It’s a good idea to replace all of these vacuum hoses, at this time, regardless of the test results. If still no vacuum, we will need to actually access the control switch in order to test the input manifold vacuum hose and the electrical function of the switch itself. As noted, on this particular model, this can be a bit of a task. Better access to the switch can be had by removing the air filter box, mass air flow sensor and throttle body (and all of the associated hoses, etc). This will allow you to access the control switch from under the manifold. Go to step 8.

NOTE: If you can access the electrical harness plug on the control switch, prior to (or instead of) disassembling the intake system, test the harness plug for 12 volts of input. Wire colors vary depending on the model (see wiring diagrams in the applicable Bentley repair manual), however, of the two wires in the harness plug, one will be brown with one or more color stripes, while the other will have no brown. The wire that does not have any brown is the input wire. Test for 12 volts from this wire to ground, with the ignition key on. If no power is found, go back and re-check fuse #2. Once power is established, continue to step 8.

8 Remove the input vacuum line from the control switch. Use a vacuum gauge to test for manifold vacuum, through the input hose, with engine running.

If no vacuum, or the reading is low, replace the input vacuum hose and retest. (Again, it’s a good idea to replace this hose no matter what the test indicates.) Go to step 9.

9    At this point, if you can access the vacuum control switch for testing (and you have verified that there are 12 volts of input), apply vacuum to the input nipple on the vacuum control switch (using the vacuum pump noted in step 5).

The valve should be closed and the vacuum pump should hold vacuum. Apply 12 volts across the two terminals of the vacuum control switch, using jumper wires. The switch should open and release the vacuum from the vacuum pump. If the valve does not function as noted in this test, it is faulty.

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