I did all kinds of searches for heaters shutting off when it is put into the 4th(or high) selection. I have yet to find a thread that has this issue. If anyone out there has had this happen could you please post up what you did to fix it. I have a 2004 Blazer 4.3L 4x4. I have checked the fuses and all are good. Numbers 1-3 work fine but 4 shuts the heater down. There is nothing coming out of the vents at all on the fourth setting. Everything else works on the heater control panel. Thanks in advance.

 

I've had this problem on a Dodge Dakota. It was the fan speed resistor. On the Dodge, it was mounted on the firewall, no idea on the location in the blazer.

 

The fan speed resistor is usually mounted on the heater box with the coils reaching inside the box near the fan motor for cooling. Look for a plug near the fan motor that goes to a flat square with one or two bolts holding it in. Sorry I can't tell you exactly where either as I have not had to replace mine yet.

 

generally when the resistor goes the fan only works on high

 

^ I agree. If high speed doesn't work, and all others do, the relay is most likely the problem. It has 5 terminals and is either in the underhood fuse panel, or close to the blower resistor, usually on the firewall. Wire colors are: Purple, black, 2 orange, (one slightly larger gauge than the other) and bark blue.

 

Thanks for the info, when I get time tomorrow, I will pop the hood and check voltages over the lines and see which one is bad then test the resistors. Thanks again everyone for the speedy response.