Cars with manual air conditioning have a resistor to control the fan speed. The normal symptom is the fan only working when the switch is set to max
Cars with climate control have a control unit (sometimes called a "resistor" when buying off ebay) The normal symptom is the fan control having no effect and the fan runs either at full speed or not at all.
Cars with auto air conditioning unit (climate control) may find that, whatever the settings are, the fan is always on (sometimes always off) irrespective of the fan setting. This can be cured by replacing the failed fan control unit that sits in the side of the heater.
To replace the fan motor control unit takes about 10 mins (in theory). The only tool needed is a T20 torx screwdriver.
Remove the right-hand covering panel (drivers side on right-hand drive cars)
Cost of unit from Peugeot £130, off ebay £20. The unit is on the drivers side slotted into the side of the heater
The FET can fail either open-circuit or short-circuit - but reflow the solder connections on the other side of the board first just in case
I changed my heater controller as the original went open circuit so no fan (odd that the known faulty spare I had was short circuit so full fan).
Job seems quite straightforward except that the plug/sockets are in VERY tight and the new cheapo Chinese sockets needed a little chamfering to get the plugs in as they are such a tight fit
. . . and refitting the little cover panel is an art in itself - took me much longer than the allotted ten minutes!
but a satisfactory outcome . . . eventually
Cars with manual air conditioning unit may find that, whatever the settings are, the fan only runs with the switch at full speed.
As above - remove the right-hand covering panel (drivers side on right-hand drive cars) to replace the failed fan resistor unit that sits in the right side of the heater.
also be aware that the plug into the top of the fan - accessible by removing the left hand glovebox - can come loose. The power cables to the blower motor normally overheat at the connector before the blower fan itself fails. Pull them out and solder two thick power lines to bypass the connector to rid of this persistent issue.
It has also been noted that the heater resistor can develop bad contacts due to arcing caused by heavy currents during switching.