Nissan Sentra 2004 heater occasionally stops working

Our Nissan Sentra 2004 heater occasionally stops working, usually when its very cold of course. Usually, the heater starts working fairly quickly but then starts blowing cold air. If you move the vent dial to different positions something changes and the heat starts working again. A mechanic says that something in the heating vents has probably come lose and to find it quite abit of the engine parts will need to be removed resulting in a big bill. Yikes, does anyone have any other ideas or have had a similiar problem?

The first thing you need to tell people is what kind of shape your cooling system is in. Has it been serviced as recommended? Has it been serviced recently? Or drained for any reason? Have you at least checked the level of coolant? Do you have any coolant leaks? Do both of your heater core hoses get hot?

Of course, the part about fiddling with the vents does suggest a vent system issue and this is the kind of thing that requires lots of labor to inspect/repair - lots of disassembly as noted. It shouldn’t be engine parts so much as interior though.

Many of these systems are controlled by engine vacuum so there are little hoses from the engine going through the firewall and actually ending up at the back of your heat controls. If you go to an auto parts store & pick up a repair manual for the car (about $20) you may be able to figure out how to check that stuff out yourself.

Nissan and Toyota like to use a vacuum controlled solenoid to open and close a valve to let coolant flow into the heater core. Under the hood, you should be able to find two heater hoses that run to the firewall. They feed engine coolant to the heater. On one of the hoses, there is a vacuum valve, and the vacuum is controlled by an electrical switch.

On my Toyota, this switch went bad, and the valve couldn’t open, leading to no heat. I found one solution to just by-pass this switch, and run the vacuum to the vacuum solenoid. The solenoid will open whenever the engine is running, but I have heat now. The heat output is controlled by a blend door, so coolant always flowing through the heater is not a problem.

Thank you for your reply. We did check the two hoses and electricl switch which were clear and working, although I’m going to check more into this switch bypass.