I’ve had this irritating problem with the gearshift on the Cayenne since I bought it. The manual shift on the Tiptronic shifter/gear lever doesn’t work, but the steering wheel buttons are fine. After a few miles driving, the gear display in the gauge cluster will default to ‘D’ and no gear is shown when using manual shift.
You want to see the gears and shift a Cayenne manually, as driving along in D is the least rewarding way to go. Porsche Tiptronic is all about performance with convenience and when it’s working, it does well with the V8 engine. Shifting on the buttons or the stick as you exit a corner or approach an ascent is the way to keep these things cooking: waiting for the ECU to decide to shift down or kicking down and burning needless fuel is hopeless.
I found this thread on Rennlist where two pointers were raised: magnets on the shift cover or water in the footwell. I stripped the entire centre console out and had a look. Turned out my problem was neither: the wiring to the shifter was chewed up, for some reason. I spliced in some new stuff, heatshrunk over it and put it all back together, giving the centre console gubbins a good clean at the same time.
Starting the Cayenne up, the display worked and shift looked good. Manual was now appearing in the cluster and the stick was working well. I call that a win: maybe it will solve some other stuff too.