2012 TD V8 Westminster - gear box selector/gear box issue
Gear box selector seems to work correctly (not sticking etc)
When selecting either R or D there is no traction and the light next to the gear on the selector flashes.
The dashboard gear indicator also flashes. Then gearbox warning appears.
Let the vehicle warm up and select again (sometimes takes a number of tries) and traction is obtained.
Seems to happen only in a morning on first start up. Later start ups appear OK.
Occasionally, when driving it seems the gearbox either unexpectedly shifts gear (up or down) or “slips” ( I can only describe it as similar to clutch slip on a manual box).
GAP tool indicates fault: U0402-64(2 “invalid data received from transmission control module-algorithm based failure-signal plausibility failure”.
I don’t really understand this - any advise appreciated. Thanks

To save a lot of messages back and forth I'll trot out a few things I did when faced with a similar problem.

How many miles on the vehicle ?
Has the transmission fluid ever been changed to your knowledge ?
Do you have a transmission fluid leak ?
Any burning smell ?
What colour is the transmission fluid (Lord knows how you'll find out without dropping the pan, maybe try to suck a bit into a tube from the fill hole !)
Check the level of the transmission fluid whilst you have the plug out - refill if necessary.
Try a tube of Dr. Tranny (yes it's a real thing).

My experiance was; Dr. Tranny fixed it for a while but it came back. Untimately had to go for a transmission swap - which was waaay expensive, I had it supplied and fitted by an independant. Looking back I should have bought one myself and fitted it - would have saved a fortune !
Good Luck Previous life - 2012 L320 SDV6 - Got divorced, say no more!
Previous life - 2014 L494 SDV6 - Got nicked, without keys, watched it go !
The Duchess - 2007 L322 TDV8 3.6 Autobiography with 270K miles on the clock
The Bonkers One - Audi SQ7 V8 4L TT - It will pull your face off.