Gearbox fault P1779

My '96 XK8 is intermittently showing ‘Gearbox fault’ and going into limp mode. The code is ‘Transmission fault’ P1779. Car has 86,000 miles on it. Could this be transmission sensors or could it need fluid?

Terry Pattison

I have had some experience in this sector. Sadly not good experience. I have replaced and/or rebuilt a total of 3 of these gearboxes in a 97 & 99 XK8, and from a 00’ S type all of which share same gearbox.
If you have no other symptoms for example harsh shifts into Drive or reverse, it may be something as simple as low battery voltage which can cause erroneous codes to present themselves including ‘Gearbox Fault’. I hope that it’s something simple for you.

Could be speed sensor or forward drum failure?

5HP24_97.pdf (68.8 KB)

I agree with James; have the battery tested first.

I had a check transmission light come on in my XJ40, i serviced transmission resulting in a back to normal operating system.