Hi, all,

I have a 2000 C230, 11K miles. Several days ago I found out my car can only blow cold air no matter the temp setting is,except I put the climate control dial on defrost. Both drive and passenger side act the same. Which is not a big problem yet since it's summer now.

Since I can still get heat on defrost, the heater cores are probably fine. I did some search and think it could be the duo-valve. I unplug the duo-valve, full heat will come since it will be open by default. I test the voltage on the three pins which operates the valve. On defrost mode, the voltage is 9.24 constant; 13.86V constant on all other temp setting, which I think it should be intermittent depend on the temp setting.

So my questions are:

What is the correct voltage reading on these three pins? Shouldn't I get 0V while setting on defrost and 12V intermittently on other setting?

If the voltage reading is wrong, does it mean whatever controls the valve went wrong?
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Update on June 13:

opened duovalve and cleaned it, everything looks fine and clean.

what do I do next?
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Will a bad coolant temperature sensor cause no heat?
Any help would be appreciate.

If you unplug the heater valve, you should get HEAT.

If not, either the heater valve is STUCK or the heater core is plugged.

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Sounds like the opposite problem I had with my son's 2000 C230K. Hot air all the time, no cold air yet the compressor line was ice cold.

New heater valve did the trick. I had a new one put in but apparently they can be cleaned and put back into service.

I did unplug it, and the heat came back as it should.