and it's hardwired into the aircraft power.
In which case it is probably an illegal installation.
I wouldnt just accept what is in the aircraft either, it is your responsibility as commander to "accept" the aircraft or otherwise. I wouldnt "accept" an aircraft with a "TAS" installation that is giving as many erroneous readings as you suggest - I would want the unit placarded u/s.
However in the event I would have the operator replace the unit as it is clealry faulty and not representative.