Nissan Repair: 2003 sentra no heat/head gasket, heater control valve, head gasket

Question
I have not heat in sentra..took to mechanic changed t-stat saw that the radiator overflow was filling up..decided it was head gasket,,brought to another mech..he supposedly did carbon test and said they were high..he also thought head gasket..I have ho heat and i believe i am burning off coolant..the car never even runs hot..The third mechanic could not even find anything wrong and got the heater to work by blowing out the system..no it doesnt work again..no heat..heater core ,water pump were checked all good..but yet i am getting some back pressure into my overflow tank..and i get no heat..I need another opinion.HeLP!

Answer
Hi Den -

If I understand correctly, your heater is not working? No hot sir blowing into the car?

Have you checked your heater control valve? This is an in line valve located alonog one of the heater hoses, usually run by engine vacuum. When you turn the heater on, it provides vacuum to open the valve. Perhaps the vacuum hose came off, or got damaged.

Please check that -

Hope this helps -
aloha
calvin