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Juan Crespo
Juan Crespo, ASE Certified Technician
Category: Audi
Satisfied Customers: 1515
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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Audi: XtraTech same car and problem. I found a vaccum leak

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XtraTech same car and problem. I found a vaccum leak at one of the IMT actuators. Repaired leak, so now we have about 20 " Hg vaccum. Tried to confirm that the combi valves were working, disconnected secondary air pump hose and blew in it, felt like both or one of the combis was open. Cycled vacuum, 17" Hg with a hand pump, to each combi, no noise opening or closing, and no difference in resistance when blowing in hose at air pump supply, (discharge of pump). Drove the car a couple pof times to get the readiness tests reset. Code 1423 came in this morning on a "cold" start. I checked the vaccum to the combis during hot starts and normal running, they are not called for. Is this what delays the seconday air system readiness test from resetting? I am therefore assuming that the combis are only use on a cold start, and that the secondary air system test saw that at least one combi was not working. How can it see that? There's no pressure sensor on the system, and no limit switch on the combi. All it can see is the individual O2 sensors on each side. Can it tell that one combi is not working and thereofer not getting the correct O2 reading? I am trying to understand why I get the fault code, and if it's worth removing one or both combis. They are not easy to get to to remove. I look forward to your reply.

Welcome back!

The secondary air system is activated on cold starts. The PCM expects to see a certain value from each bank's oxygen sensor that is predetermined by the 'cold start' algorithm. If those values are outside the limits, a code is triggered and stored as 'pending' until the next consecutive occurrence when the check engine light comes on. Readiness tests won't run until all codes are cleared.

It is very common for these systems to fail due to carbon build up. AIR pumps, combi valves and vacuum hoses are also known to fail quite often.

Customer: replied 3 years ago.


as I have inspected the hoses, and determined that the pump does run on a cold start, and that I have only one code, 1423, which indicates bank 1, would you agree that I probably have a failed combi valve on the passenger side? And that it would be worth removing the valve and attempting to clean it to get it working?

Yes. Here is what I recommended last time:

"Once the vacuum lines have been replaced and the A.I.R. pump is confirmed as working OK and not screeching like a banshee, the next step is to check the combi valves (in your specific case, start with the one for Bank 1).
You can try and check their functionality while still on the car, but this may be hit or miss. You need a hand vacuum pump to do this. Use the vacuum pump and pump it up to about -20 "Hg. If you suddenly release the vacuum, and can feel/hear a nice little thunk from the valve, the diaphragm is probably fully opening and closing. If you don't hear or feel it, it could be sticking. What you want to do then is remove the combi valve from the car. Use some brake cleaner, and spray it out with the valve both open and shut (use the hand pump). You should have a lot of carbon build up wash out. Operate it off the car and see if you can get smooth operations in the opening and closing. If you do, the valve is probably fine and you saved yourself from buying a new valve. If you are at this stage, it's best to engage in the next repair I have listed.

Once you take off the combi valve, use a coat hanger to clean out the carbon in the air passage; then use brake cleaner to clean the passage even more. Brake cleaner does a great job at dissolving carbon. Use some spare vacuum hose (hard nylon line if you have it) to shove in the passage. Use the straw on the brake cleaner can, insert it in the hose, and spray away. Follow up with more coat hanger cleaning. Be careful not to break anything off or get something stuck in that passage way. If you do, you'll basically have to pull the head to try and get it out of there."
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