I thought it might be useful to other members if I relayed my experience sorting out a problem on my R170 whereby the driver's side would only blow out hot air regardless of the setting of the temperature control. The passenger's side worked fine.
Not knowing where to start I search this excellent forum and found the postings explaining that this normally caused by a failed duo-valve.
Following the suggestions I disconnected the three way plug and put a multimeter set to DC volts between the centre pin on the cable socket and first the left and then the right pin. Each time I cycled the temperature control from max hot to max cold. This showed that each circuit went between zero and 12 volts, an excellent result as this proved that the control unit in the dash was not at fault.
I then removed the top half of the duo valve see this excellent description:-
I then set the multimeter to resistance and measured between the centre pin and each side and found that one had a reading of around 2k ohms where as the other was over 5 meg ohms, a simple ohms law calc shows that this wont pass any meaningful current so chances are the coil had become open circuit and the high resistance reading was just coming from the capacitor that is across each coil.
I rang MB to see if I could just buy a coil...fat chance. No I had to buy the whole assembly at over £200 inc VAT. I did however gleam the part No.
I quick Ebay search turned up a number of duo valves but with a slightly different part number i.e. 0018303484 not 0018302484. On the basis it was just £10 I thought I'd take a chance.
Part duly arrived and the differences became apparent, although the top of the valve, i.e. where the coils and valve actuators are, is the the same the lower half has the hose connectors facing 180 degrees round and so would be doubtful if the hoses would reach.
No problem I simply swapped over the top half and all is fine.
A £200 saving and a warm glow of satisfaction of 'beating' the MB system.
Hope this helps