Dont know about OP's car, but I have found that when older diesle Peugeot come up with "Depolution fault" and a little picture on the dash display, as opposed to just a MIL on, especially at around 80,000 miles. It typically refers to the need for the DPF additive tank to be refilled
(A bit like topping up ad-blue, but something that only needs to be done every 80,000 miles or so on the Peugeot system)
It is a nasty messy job involving unpleasant chemicals being squirted into a not verry accessible tank under the floor but it is DIYable provided you have acess to a suitable scan tool for resetting the counter.
To put this job off for as long as possible, run the tank all the way down and refill to brim each time you top up.
The counter operates on the number of fuel flap operations, Bit like redex, you get a squirt evey time you open the flap so you will use the stuff up much quicker if you fill up little and often.