OBD. Not able to read computer on car

Got a restricted performance message on dashboard of out jaguar xj sovereign v6 TDVI 2008

it drives fine !!! Trying to get it read to find the fault is frustrating !! Garage cannot read it though they said they had previously but not can not!

a diagnostic wizard looked at it trying to solve it for an hour came back with one of the pins in the connector under the steering wheel is not earthing so asked another specialist about what to do he came back with but an ecu. But if it was the ecu it would not work like it is !! You google the faults of sun ecu failure and the main one is the check engine light does not light up car does not shake or stop’s unexpectedly no drop in fuel economy!!

The only thing that happens to say it has restricted performance is there’s no power to overtake !!

it drives at 30, 50 , and even 70, with no restriction except for no extra power for overtaking.

Also the light now appears within minutes from first start not like a couple of weeks ago that it used to come on after around 10 to 15 miles into a journey 

it was intermittent until 2 weeks ago.  A month previous to that it came on as we left for a break in Scotland  drove it a while left it over night went off a snd never came back on until 2 weeks ago 

can anybody help with this very puzzling situation I have with my beautiful Mighty Cat 

Regards Suzie