Heat only works first time in the morning
 

2000 C230 Kompressor 110K

My car has dial for temperature control. It blows full cold air no matter the dial setting, except on defrost, which indicate the heat core is not blocked. I remove and cleaned duo-valve, it looks fine. 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.

For the last two days, I found the climate control is normal when I start the engine in the morning. It stays that way until I stop the car and restart it.

I have a coolant flush several weeks ago and I also change Engine Coolant Temp Sensor and thermostat three days ago(I thought it might be the problem and does not cost much and also never been change).

So, guys, any idea that the problem might be? Thanks.