HI
If the cts voltage goes down as temp goes up and you are seeing incorrect readings on your scan tool you need to measure the voltage at the ecu plug to see if the same as at the cts sensor.
if the same and correct for the temp car is at then ecu fault.

Also a 2001 car may take a software patch repair...reccomend to your customer to check for an ecu update check with your peugeot dealer first.

update to latest software then if you still have fault its a hard car fault and not software related trust me been there

simon