I'm posting this because I had no luck at finding information in the forum to assist in solving the problem of our Jetta being in transmission Limp Mode. Limp mode results in the transmission operating only in 1st and 3rd gear, with upshifts at about 4000 rpm.
Our 2001 AEG with 01M automatic transmission went into limp mode and showed OBD code P0722. Bentley describes P0722 as transmission output speed sensor. The weather was bad, my wife was screaming, so I took the car to the dealer. They called and said the car ran fine on a test drive, but still showed a DTC for Vehicle Speed Sensor. I paid them $150 to replace the sensor.
A couple weeks go by and the car is back into Limp Mode. I start researching it. The 01M has three speed sensors on the transmission. These are identified in the wiring diagram as G22, G38, and G68. G38 and G68 connect through the harness to the Transmission Control Module (TCM). G22 connects to the back of the instrument cluster. My professional VW mechanic, with a shop full of manuals, TSB's, and computers, had replaced the speedo sensor. I went back to the dealer and had this conversation:
Me: There are 3 sensors. Two go to the TCM and one goes to the speedometer. The complaint was transmission in limp mode and you replaced the speedometer sensor. I don't think that's right.
Dealer: These things are all interconnected. It could be the speedo sensor causing the transmission problem.
Me: Then why is it doing it again? Doesn't VAG-COM tell you which sensor is bad?
Dealer: We can't tell you anything unless you bring it back in and let us do a diagnosis. That will be $110.
Me (unspoken): When pigs fly, you ******!
VW Technical Bulletin C01-99-02 addresses the issue of speed sensors and DTC's. It shows the location of each sensor and identifies it by the wiring diagram "G" number. It also tells which sensor causes each DTC, but it is listing VW DTC's that are not cross-referenced to OBD codes in Bentley. It also implies that the VAG tool shows the wrong sensor for DTC-00281. Bentley also has illustrations in the AT section to identify the sensors by the G number.
The problem then becomes to determine which of the sensors is associated with the P0722 code. P0722 is "output speed sensor," but nowhere are any of the sensors described with those words. My successful repair would indicate the following:
The G38 sensor, on the center top of the transmission, directly accessible under the battery tray, is apparently the input speed sensor referenced by OBD code P0717. The G22 speedometer sensor is on top of the differential housing, in a silver heat shield behind the engine. The G68 sensor is the output speed sensor referred to by code P0722 and is on top of the transmission, under the mount support bracket.
Bentley makes the replacement of G68 sound terrible. It is not. Remove the air cleaner housing and battery tray. G68 is out of sight, under the transmission mount support bracket. Support the weight of the transmission on a jack, lifting on the skid plate. The support bracket must be removed. VW says to replace all 4 support bolts (these are the same bolts replaced in a timing belt replacement). Instead of replacing them, before removal I used a Sharpie to make match marks on all 4 bolts. When replacing them I realigned the marks, plus 1/12 turn. If the transmission falls out, I'll be sure to post it.
With the 4 bolts out, lower the transmission about 2 inches to make room to move the support bracket aside. You don't need to get it out of the car, just move it aside so you can get to the speed sensor. Everything from here is simple remove and replace.
Buying the sensor was something of an ordeal. The parts guy brought out the G22 speedo sensor (357 919 149B). If I had not had the Bentley manual and VW TSB with me, I think he would not have believed that the other sensors existed. The G68 output speed sensor has a pigtail harness; the G38 input sensor does not. My G68 is 01M 927 321B. My G38 is 095 927 321B. Check your numbers; the parts guy said there was a G68 number change on later models.
It's now been about 3 months, and everything is good, so I believe this info is right.