Whilst the Q unit can be used as a replacement for an E or P unit, E and P units cannot necessarily be used as a replacement for a Q unit.
The reason for this is that the E and P units lack the internal hardware to control rear doors with manual windows. It only has the hardware to work with rear electric windows. The Q unit has the hardware to work with both, but you have to tell it which system it's working with by way of a software adaption. Failure to do so results in conflicting information about the position of the microswitches in the rear doors being broadcast on the CAN network.
Q units were fitted in cars that left the factory with manual rear windows. They are therefore relatively common, but they're almost invariably set up to work with manual rear windows. This must be changed if they're to be used as a substitute for an E or P unit.
All CCCU variants mentioned here work with the same remote key fobs. The later, more reliable generation of CCCUs has its own cross-compatibility limitations.
Cheers,
Tom
