On Tuesday upon leaving work, I started the car, backed out of my parking space and upon shifting to drive, found the car would only creep forward. I looked down at the dash and there was the dreaded "gearbox fault" message. I had the car towed home. Next morning I hooked up the scanner to check for codes (none) and the car moved fine in the driveway (I still haven't driven it on the road till I can check thoroughly.) I've been having an intermittent starting problem and I've read that low voltages can cause all sorts of weirdness. Here comes the questions that might help me understand where to go with this diagnosis.

What triggers the "gearbox fault" message and what is this "limp mode" that apparently lets the car move under very low power? Is it a mechanical function within the valve body that is triggered by a loss in hydraulic pressure and "sensed" by the computer or is it a decision that is made by the computer itself to electronically put the transmission in a "limp mode". OR as much as I've read about the A-drum problems, is the "limp mode" merely a function of the broken A-drum not letting all the power get through to the wheels?

 

The dreaded gearbox fault message ! I had the same thing while on a high speed run across country, had to get lifted onto a breakdown truck and taken home. Got the car off the truck, started it, put it in gear and it drove fine without any problem. The only fault code was for an abs sensor which was changed and the gearbox stayed good.
I think that the Jag computers don't want you to drive the car if anything safety related has gone down like an abs sensor so it stops you driving the car by telling you lies.
It could be something really simple so don't rush out and buy a gearbox until you have checked the codes and rectified any small problems first.

 

I had the same problem, the shifter cable needs adjusting. Before doing the adjustment, I had to slammed the shifter into D, in order not to get the message on the screen.