thanks for suggestion,
the garage says if it was the sensor it would show that as a fault. Mines comes up with what they call a unknown code. This happened on my own garages diagnostic tool and then again when it was taked to a peurgot dealer. I wondered if factory would be able to decode it but they had not recorded the fault number.
They say it will not effect the MOT so I am learning to live with it.
I can understand some faults not bringing up faults numbers but not unknown numbers soming wrong with their system?