How do you know if the problem is the fan motor or the resistor when the heater fan on a Honda civic 1990 won't work?

Pull the connector off the blower motor and put 12 volts DC onto the two metal tabs that feed the blower motor. If the motor does not run with 12 V DC applied, replace the motor. If the blower motor runs when you put the 12 V DC on it, then the motor is OK, your problem is elsewhere. Some cars have a power resistor between the fan switch and the blower motor. This could be your problem. Check the connector feeding the power resistor. If you don't get any voltage feeding the power resistor, your problem might be the fan switch itself. If power gets to the power resistor but not to the blower motor, it is probably the power resistor.

If your fan is only working on 2 speed (2 highest speed ) That means that you blower Fan RESISTOR is broken , you can fix it by removing the little box just on the top of your fan motor (on the white casing ) behind the glove box. There is some wire connected on it (blue for me) . You need to unplug the wires , and unscrew the star bolt. You can now remove the resistance circuit and see what is broken. If the wire are broken you will Need a 3 , 8 and 12 ohms resistance .

M2 TO HIGH : 3ohms

M1 TO LO: 8 ohms

M1 TO M2 : 12 ohms

This will make you save money cause youll not need to buy the 40$ resistance circuit at you local Honda retailer.