Power Output Stage Diagnostics and Solutions
 

Our AAN, ABY and ADU-powered engines use a coil-on-plug ignition system comprised of five coils and two final power output stages (POSs) . As part of this system the five coils are powered hot when the ignition system is one. However, the coils are not grounded until the ECU decides that they should be, at the appropriate moment, to create the high voltage spark on the secondary side of the coil. This switching to ground is done via transistorized switches known in Audi-speak as Power Output Stages (POSs). In our case, they are devices N122 and N127.

As these components age, it is possible to have failures on either (or both) the coils or individual channels in the POSs. Coil issues usually start making themselves known at high boost, e.g. WOT, whereas POSs channels can cause a miss even at idle. Sometimes coils miss at idle too. Or it can be the plugs (damaged, old, not tight) or the coil to plug boots failing. Here is an image of the diagram of the general components in question (coils are Item 1; POSs, Item 15; coil to plug boots (connectors), Item 2)

http://12v.org/urs/AANignitionPNs.jpg

The two POSs are designated N122 and N127. Each has a four pin connector from the ECU harness and a three pin connector to the coils. N122 serves cylinders 1, 2 and 3 and N127 serves cylinders 4 and 5. As a result, although the N122 connectors are fully populated with female pins, the N127 connectors are each missing a pin (for the "phantom" cylinder "6").

http://12v.org/urs/N122andN127Annotated.jpg

If you had X-ray vision and a labeling gun, the N122 and N127 would look like this (Note: Wire colours are based on the AAN and might be different for the ABY and ADU):

http://12v.org/urs/N122Annotated.jpghttp://12v.org/urs/N127Annotated.jpg

The standard method to find a miss is to remove the plastic injector cover and start the engine. With the engine idling, you pull the electrical connector on Cylinder 1. If the missing (engine shaking) at idle, gets worse, the problem is NOT Cylinder 1. Replace the Cylinder 1 injector connection and move to Cylinder 2 and remove its connector. Repeat until you have confirmed that the miss is on a particular cylinder. You will know this because when you pull the electrical connector on that cylinder's fuel injector, the miss (shaking at idle) will NOT get worse. The problem is you don't know whether the issue is a POS channel or a coil or a plug or a coil to plug boot.

Now comes the part where you can confirm what kind of fault you have, coil/plug/boot or POS channel.

What you need to do is to swap the N122 and N127 connectors in pairs, as shown here (turn off the engine to do the swap):

http://12v.org/urs/PreSwap.jpghttp://12v.org/urs/PostSwap.jpg

When you have made the paired connector swap, you repeat the procedure of pulling the injector electrical connectors to find out where the miss moved to (or not, it might stay where it was). Here is the results table:

http://12v.org/urs/POSDiagnosticsTable_Rev1.jpg

If you need to investigate the coils or plugs further, you could look at this missing and hesitation post

If you need to swap the pins, you could look at this POS pin swapping hints post.

232 views and 0 comments. I guess I was perfectly clear. ;>)

It is an impressive post, with all the great diagrams and all. I would also guess its one
of the more common issues now, with our aging UrS-cars.

I would have been all over it this time last year, when I had a failing POS. I ended up calling
Marc and going with the LS2 kit after your input on my symptoms, which worked :)

///

You just saved me a bunch of unnecessary work. THANK YOU. Every single week I am under the hood of this car. Fix one thing, and now a horrible misfire.