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.