Car is a 92 400E. AC normally works great but a couple of times in the last week it wouldn't work - just blows hot air. It seemed to start to work and would then cut out. There's a fault code 6 on the Base Module which is "Electromagnetic A/C Compressor Clutch Blocked". No codes at all on the AC socket. So what are the possible causes of a "blocked" compressor clutch?