Heater fan only works on fastest speed

Both fans come on.

Cars usually overheat when the thermostat is faulty, coolant level is low, or the water pump fails. If your cooling fan on the radiator does not kick on shortly after the engine reaches operating temperature, you may have a bad cooling fan, cooling fan relay, or cooling fan fuse. If your engine is overheating, start by checking the fluid level, and installing a new thermostat.

A blower motor resistor is the device that controls a car's cabin heating and air conditioning unit's fan speeds. This usually won't cause the engine to overheat, your A/C fan may not function or only function at one speed. Here is a Blower Motor Resistor from Autozone: http://www.autozone.com/autozone/parts/2003-Ford-Focus/A-C-Heater-Blower-Motor-Resistor/_/N-jk8uvZ91tag

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low speed fan relay is bad

leaking coolant has nothing to do w/heater fan. maybe just bad switch. check fan switch.do need to fix coolant leak to.

Bad blower resistor or switch.

common problem, replace the blower motor (fan) resistor, this is what controls the fan speed,its located on heaterbox next to blower motor.heres a picture of it,its on the rightside of dash toward the bottom. hope this helps.

change heater control switch common problem, ive had to change mine twice

sound like control AC + HEATER control switch is faulty.

This sounds like the blower resistor has gone out. This is the component that allows the proper voltage to the fan to change the speed. Have it checked and replaced. That should get you back in business. Hope this helps. Good luck.

change the fan resistor,its under passenger dash,very cheap to buy and fit in 20mins.open your dash and press 2 side in to let dash door drop down.you,ll see it on left hand side on a left hand drive car and right side on a right hand drive.it contols the fan speeds.just release wire block,undo screw,take out and put new one in.normaly white or blue in colour.hope this helps