Heater not working, 93 Maxima GLE

[billbill2k]

Hey all, hope you can help me. I have a 93 Maxima GLE with an automatic air conditioner. The air conditioning works fine, but the heater is providing no heat whatsoever. Was able to get it working manually once, but find this very difficult to repeat. Does nayone have any idea what could be causing this, possible remedies or methods of diagnosis? The coolant valve seems to work properly, and also the diagnostic codes on the display dont seem to be working. Thank you all, any help would be appreciated! -Bill