There are many of us facing the dreaded CPS DTCs, and there are hundreds of threads or hits on Google. Let's see if we can pull our collective knowledge together and help everyone, including me.

Here are the typical troubleshooting steps:

1. Check your timing belt. You may have slipped a tooth, have a worn belt and weak tensioner. Simply put, it's TB time. Important also to follow the Bently repair process with the cam lock bar!
2. Be sure your wiring is in proper condition. The key tests: 5v between pin 1&3 on both G40 and G163 connectors. Next, test pin 2 for voltage with key on and car not running its ~11v, and while running it's 12-13v.
3. Check for shorts. Confirm you have ~1.5Ohms between G40 Pin2 to ECM pin 87, and G163 pin 2 and ECM pin 86. You can also test G40 pin 1 to ECM pin 98. Be sure to also check for shorts between all pins and common ground.
4. If you are only getting one of the two codes, swapping the CPSs to see if the code follows it has been one trick mentioned.
5. Vag Block 90 I've seen may give readings for the two sensors, but I haven't found the details. When I've tried, both show 0.
6. Some have blamed the problem on the ground wires at the back of the motor connecting to the firewall. Undo the 10mm bolt and sand/clean and reassemble.

A couple useful facts:
G40 failure = P1338 Bank 1 - Open Short to Plus
G163 failure = P1392 Bank 1 - Open Short to Plus
G40 location, front right cylinder head
G163 location, left rear cylinder head somewhat under radiator res

Now for the hard part. I've done all this and I still get the errors. I haven't replaced the CPSs, mostly as it's hard to believe they've both gone out at the same time.. My searches also suggest replacing the ECM !! Wow, really..has anyone had to replace the ECM to rid this problem??

I did the TB, and it's spot on. I've checked all the wires, no shorts and good connections with the ECM. I've run the engine with G40, then G163, then both disconnected.. no change to codes which returned (both). Next I guess is to replace both the CPS.... and fingers crossed it works.

Lastly, there seems to be much confusion as to the purpose of the CPS. Some have said they are only used at start up, others that they are involved in engine management while running. All I know, I can start the car with both disconnected (only if both codes are in the ECM), and I can run the car with one or both disconnected. So WHAT are they really doing... other than throwing the codes preventing me from passing inspection!

Please Help.... Thanks!

Some additional useful tips:
Does your Audi UrS4/S6 sometimes fail to start but once running seems to run fine? Your G40 Cam Position Sensor (CPS) may be starting to fail.

On the S4/S6 (and many other 90's era Audis) the cam position sensor is only used during startup to tell the engine when the compression stroke occurs. from then on out, the speed sensor (flywheel tooth counter) and reference pin sensors take over. The result is that if you have a flaky cam position sensor, the car will sometimes not start until a signal is sent from the G40 CPS.

The CPS is a hall sender, which works by producing a 5v pulse every time the "window" on the shutter wheel behind the camshaft exposes the hall sender to the receiver. this pulse is then received by the ECU to tell it when the compression stroke is occuring.

To "see" this pulse, you can connect an LED between the signal and ground pins of the connector, which on the UrS4/S6 is pin 3 for ground, and pin 2 for signal. Standard LEDs are only rated to work up to 2v, so you may want to use an inline resistor (its also possible to buy LEDs with the resistor built in). Remember you're dealing with 5v not 12, so you only need about a 150 ohm resistor.

See link: http://www.efiexpress.com/catalog/ar...articles_id=11