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BMW MINI – P0491 and P0492, Secondary Air Flow Insufficient, Low

January 26, 2017

I have a quick question for Otto.

I have a 2000 540i.  I got the dreaded secondary air pump related codes the other day after my sap [seondary air pump] died, so I switched out the pump, check valve, relay switch and vacuum lines, but still [have the] P0491 and P0492 codes.  My gas mileage is terrible so I am pretty sure the the O2 sensors are bad.  My question is; could faulty o2 sensors be the cause of the sap codes? I have already ordered the [four] O2 sensors and tool from you guys, but I am interested in your opinion. Thank you.

Nigel,

The P0491 & 0492 codes are recording insufficient secondary air flow on bank-1 and bank-2 (Cylinders 1-4 and 5-8).  These codes are generated at the pre-cat O2 sensors, during cold start, when the secondary air pump is operational.  The sensors are indicating a richer mixture than desired (during this period) and the system is assuming that the secondary air flow into the exhaust ports is too low.  With all new parts; pump, check valve, vacuum switch, etc., the likely culprit is clogged ports in the heads.  The air is pumped into the cylinder heads at the front of the heads (where the secondary air cross-over manifold bolts to the front of each head).  The heads have ports machined through them to direct the air into each exhaust port.  It’s not uncommon for these ports to eventually clog with carbon from the exhaust port …. especially if the pump or check valve has been faulty for an extended time.

Note – This can happen with any of the BMW or MINI engine models (4-cyl, 6-cyl, V8, etc.) that have secondary air ports machined within the cylinder head castings (M52, M52TU, M54, S52, S54, S62, M60TU, M62, N62)

Sooo …. If you have tested that the pump is running and that the check valve is opening while the pump is running, the ports are likely the issue.

Click HERE to go to the BAVauto® Secondary Air System Diagnostic Guide

Other possibilities?

You noted that you intend to install fresh pre-cat O2 sensors.  While I would initially say that it’s not likely to cure the fault codes, it is possible that the sensors are just barely indicating a rich enough mixture, during cold start, that the system sees this as low secondary air flow, but once past cold start the sensor readings are within expected parameters and not throwing further codes.  It certainly would not hurt to install the sensors, clear the codes, and see what you get.  You may luck out.

You noted that you have poor fuel mileage.  If the engine s running rich overall, it could be tricking the system to think the secondary air flow is low, during the cold start.  However, if it were overall rich, it would likely be generating other fault codes indicating a rich mixture after the cold start phase.  Using a diagnostic tool to monitor O2 sensor voltages and fuel trim values may help determine if it is running rich (or even lean, for that matter).

So, What can be done if the ports are clogged?

In the end, if you actually do have clogged secondary air ports in the heads, there is no quick fix.  We have had some minor success with filling the ports with Liqui-Moly Jectron (available from BAVauto.com) carbon cleaner, letting it sit and soak, then applying compressed air into the ports.  The very basic steps would be:

1)Before starting the actual procedure, remove the secondary air check valve and devise a set-up to connect a hose to the secondary air inlet flange (where the check valve was mounted), so that the hose is ultimately connected to the air inlet port.

2) Warm the engine to normal operating temperature.

3) Remove the secondary air check valve.

4) Install the hose set-up (from step-1) to the secondary air inlet flange.

5) Apply compressed air to the hose (how much pressure you can apply will be dependent on your hose set-up) and ultimately, the secondary air ports.  This may initially clear some loose clogging.

6) Fill the ports and hose with Liqui Moly Jectron carbon cleaner.  We recommend adding a funnel to the inlet of the hose, not only for ease of filling, but so you can actually fill the funnel a bit.  This will allow you to see if the fluid is flowing through or not flowing at all.  If the fluid does flow through, only fill once, then let it soak as in step-7.  Do not continue to pour in more fluid.

Click image below for Liqui Moly Ventil Sauber:

7) Let the cleaner soak until the engine is cool.

8)  Apply compressed air, as in step-5.

9) Start the engine and run until hot.  Repeat steps 5-8 until ports are clear, or it is apparent that no progress is being made.

10) Reassemble the secondary air system.

11)  Clear the pre-existing fault codes

12) Drive the vehicle and note if fault codes return or are eliminated.  If they return, you can re-do the steps to continue trying to clear the ports.


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2 Comments
  1. Paco Moura permalink

    My 2006 BMW X3 (e83) acceceeleration stutters beyond 78 mph particularly when under uphill loads. Is this possibly loose transfer case chain? front or rear differentials? or something else entirely?

    • Those symptoms don’t seem like a transfer case issue. Have you had it scanned for engine, transmission or T-Case fault codes?

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