Suddenly my heater fan only works on high speed. What could be the problem?

maybe your blower motor resistor is not good

Sounds like the fan speed relay is giving you the problem. This is the part that contols the speeds of the blower motor by changing the resistance or the power going to the motor. It could also be the speed control switch on the control head but the resistor is the most common problem.

Most likely a bad blower motor resistor, but also check for a bad wiring connector at the resistor (now supplied with some replacement resistors.)