4.2L System too lean error code
 

I have been getting P1136 and P1138 error codes on our 2003 A6 4.2 for a couple months now. I have been trying a fix, clearing the code and after 100kms or so it returns.

So far I have:

Replaced intake manifold linkage arms
Replaced fuel filter
Removed and cleaned MAF
Checked for vacuum leaks

When checking for vacuum leaks, it appears that the Throttle valve control module J338 pulls vacuum through the port on the top. Is this normal?

I have been considering replacing the FPR and/or the MAF but would rather not spend the money and hope that clears it up. Any suggestions?

Re: 4.2L System too lean error code
 

It returns after 100km because it takes that long to trigger block 032 +/-25%.

I would consider looking at your O2 sensors, also checking for intake/vac leaks

Re: 4.2L System too lean error code
 

Is there supposed to be a leak at the throttle valve control module?

Is there a way I can check the performance of the o2 sensors without just replace them and see if that works?

Re: 4.2L System too lean error code
 

I don't have a 4.2L, but I would wager a guess that there should be no unmetered air entering the system, if you have found unmetered air entering (after the maf), I would try and stop it, then see if the codes go away.

You can use VCDS to check block 032 in engine measuring blocks, to be within audi spec, you should see +/- 7.5%, on both idle and part throttle trims. You can watch it climb or drop out of range when there is an issue