I have a 2000 blazer Lt. When fan control is put on high fan stops working. I already replaced the resistor under hood by fan. The fan works fine on 1-3 setting. Anyone help me?

 

There's a relay mounted on the firewall in the engine bay, up high near the blower motor and resistor side, that's sole purpose is to provide power to the blower motor for the high speed. It's actually very common for them to burn out. Pretty easy to find one in a junk yard and a quick and simple fix most of the time.

 

I have already relaced this. Or is there another relay i do not know location of?

 

Negative, that's the resistor itself, I'm referring to an actual relay. It's held to the firewall with a 10mm bolt if memory serves me correct.

Usually when a resistor goes out you loose lower speeds.

This picture is of a 96, some of the older ones had multiple relays in this same location, but they cleaned it up in later years and I think your 2000 should only have the one for the blower motor high speed relay still there (my 2004 has only the one). If there are multiple relays there like in this photo, I'm not sure what the others do to be honest, but you could try swapping one over temporarily as I'm fairly certain that they're all the same there and and seeing if it makes the high speed start to work again, that'll tell you if the relay is at fault at least.