My 300SD was having problems with not getting any heat. The original monovalve was a Bosch 1 147 213 007-711 that last about 10 years. I bought a replacement MTC Mono Valve Repair Kit 0008350644/64118390132 and installed it. I still had the same problem. In the other threads, I discovered that the MTC mono valves have a weak return spring. The return spring is not strong enough to overcome the pressure of the water coming through the monovalve and the suction of the recirculating pump. Unfortunately, the Bosch monovalve seems to be out of production and now very expensive. MTC is the only choice. My do not buy list of brands now includes MTC and uro. What we are going to do for monovalve repair kits?

My solution is to add an o-ring to the base of the monovalve and reinstall. Get an o-ring a little small than the base of the monovalve. Just place the o-ring centered into monovalve seat, and put everything back together. The o-ring increases the open distance between monovalve stopper and the seat. In turn, this makes it more difficult for the water pressure to close the monovalve.

I do not think this was necessary, but I disconnected the power (near the battery) for the recirculating pump to get rid of the suction. I did not like the recirculating pump because it was always running. There was probably a stuck relay in the climate control circuit board. I also put silicone adhesive around the screen of the monovalve thinking that it would keep the water from pushing down on the stopper.

I have been testing the o-ring solution for 2 weeks. My results are: No heat with air conditioner running. No heat on EC mode and max cold. 145 F degree heat from the heater at max. At 22 C degrees setting on climate control, moderated heat. I hope this keeps working through the winter.

While I do not think that MTC will ever fix this issue with their monovalve, this o-ring modification should fix the problem for nearly the price of nothing if you have extra o-rings laying around. Hope this helps those who have no heat in their W126.