Hi Everybody,
I just bought my 2000 XJ8. Only had it 2 wks. Diagnostics were ran just just couple days b 4 I picked it up. No codes and clean bill of health.Few days ago, it was cool out but when I attempted to turn heat on, it blew nothing but COLD air and then I got"gear box fault" message and it went in to limp mode. I did not drive it for a day.When I cranked the next day, message was gone and everything was fine. I tried the heater again this a.m. and again COLD air, gear box fault message, and limp mode again! Does anyone have a suggestion?

 

Look for archives here and at Jag-lovers for info on the heater pump. Likely the brushes have worn out. Much less likely, the valve could be stuck. A plugged heater matrix generally results in low heat, not no heat.

 

Also check battery isn't failing as that causes a lot of weird faults.