Gearbox fail safe mode

Hi, I have a 2004 TD6,with the GM gearbox. I have changed the oil and filter but still issuing problems. The car changes gear but when it goes to change to 4th the failsafe mode comes on and the box stays in 3rd. There was a fault code saying something about gear 4 incorrect ratio. I had a snap on guy here today but his equipment failed to communicate with the box and he thinks it's the ecu. Any ideas anyone ?

Andy.

Can you manually shift into either 4th or 5th gears? If 5th is unavailable too then it could be a direct clutch issue :

is it the dreaded box

Phil

Hi Phil,
using the manual makes no difference at all. It does it when cold so can't use the motor at all now.