My daughter complained that the heat on her 1996 C280 (250,000 mi.) was intermittent. Specifically, that it would blow warm air from the driver's side dash vent, but cold air from the center vent. It was back from college at Thanksgiving time, and I prepared by having a new duovalve in hand, which was the failure I suspected from having reviewed posts on this forum. I had her retrieve codes from the CCU in advance, which seemed to indicate a left side duovalve problem.

By the time I got my hands on the car, it was giving me a code for the auxiliary water pump as well. Of course, there was never a problem with the heat when I drove it. And the forums seem to say that there should be heat from the ends of the dashboard, but no air at all from the center vents. So I wasn't sure that there was a problem. I measured the resistance across both sides of the valve, expecting that they might be different, but they were the same. I did not want to install a $200, non-returnable part just yet.

I found a post in which Gilly explained that the watervaves are designed to fail totally open, resulting in full heat. So I decided to unplug the electical connector on the duovalve to see what happens.

Sure enough, so much heat comes out of the end vents, that the CCU calls for the A/C to get the temperature down to the setpoint. Lots of cold air from the center vents. I replaced the duovalve, and six weeks later, all seems to be well.

Maybe it wasn't the only failure that could have caused the same symptoms, but it worked out ok for me.