Yesterday while driving, I got a message saying "Gearbox fault" and my xjr went into limp home mode. I turned the car off and started the engine again and it went away. I have not seen it come back yet. Was this a false alarm or a warning?

Please list vehicle information including MY and kilometres/miles.

What is the age and condition of the battery? A failing or weak battery can cause random DTCs and 'faults' to be set. The battery should have a minimum of 12.6 volts across the terminals after the ignition has been in the OFF position for ten minutes.

New battery, 2004 XJR, 114500 miles. I will test the battery with my multimeter.

By any chance was it wet outside? Are you having any other "weird" electrical problems. I had the same problem and I replaced the battery (didn't help) and then next I decided to check/clean the grounds. The PO had replaced the ground by the RH headlights, but the ground lug near the LH was still original. I cleaned the stud, but it twisted right off with very little torque. So, I went ahead and replaced that and cleaned all the other ground points that were easily accessible in the engine compartment and trunk. Not only has the "gearbox fault" error not returning, it seems to be shifting more smoothly overall. I should add that I had a few other clues as well (slight dimming of the headlights, occasional flicker of the nav screen), so YMMV.

Hmm, not having any other electrical issues yet. It is possible that more problems could arise since the gearbox fault error was so recent, but I will certainly hope thats not the case.