I had a 2010 2.0 Fiat Scudo 120hp, if it is the same Peugeot 16 valve engine (RHR) then at the very back between engine and bulkhead I am guessing .
I had to change my heater plugs which were there and I had to remove EGR cooler and loads of other stuff to access and found a endoscopy type camera was useful as very little room.
Possibly on back of cam cover on drivers side.
Incidentally there was a very helpful piece on Peugeot Forum with video which may guide you. I think it was for that engine in a Peugeot 407.