Peugeot Repair Questions? Ask a Mechanic for Answers ASAP
I'd 1st take a look at the colour of the transmission oil it should be a dark red colour if its black then its needs changing together with its oil filter. Also check its level with the gearbox warm and just after you've moved the selector through all the positions leaving it for a few seconds in each one and then back to park.
You can top up the box through the side fill plug if required and make sure you use the correct oil for your car as the fluids are not all the same. Some manufacturers recommended a change of the type of fluid - Speak to the local dealer and parts dept - they will advise you on the updated fluid to use..
If that’s all OK then check all the electrical connections of the gearbox, remove each connector in turn and spray with contact cleaner. Check that the wiring to the gearbox is in good condition with no chafing or frayed wire
Then - you should top up if low and re try it - or, if its level is ok - and condition is ok - then the next step is to drain it off - open it up and clean the filters inside -- refill and retry it..
the oil 'sealed for life' statement is true providing that you want the 'life' to be 80K miles.....
there is a drain plug so the official way to check is drain oil via the drain plug and refill with 2Ltrs of new oil.
Filling is via the breather on top of gearbox
I'd agree that 18K miles is a bit low to need fresh fluid if all was well
unless the transmission has been running hot or slipping the clutches then it should be ok
however your description of the issue makes it sound like it does have an issue and DCT (dual clutch) transmissions are very sensitive to oil quality
its a characteristic that can effect the early cars but this was corrected with code updates pretty early on and not something you tend to see on more modern ( last 5 years) cars
don't forget to ask about the latest transmission code update