Discussions

the blower motor resistor. Only running on the highest speed is a common symptom.

The GM vehicles I've had in the past had a resistor card close to the below motor that allowed the motor to run at different speeds. This resistor had two screws holding it in place and was connected to the electrical controls from the heater. If any of the resistors on this card failed (usually due to over heating of the resistor), that speed would not work. Top speed would work because at that speed, no resistors are used to lower the speed.  
 
Here;s a video showing disassembly on a Tacoma where the owner replaced the resistor AND the wiring harness:  
 

Dirty contacts on the heater fan switch id bet money on that  

great if you can diy it but do shops now a days really just replace crap they "think" might not work? what im saying really is a good shop would trouble shoot the issue first and should know the most common issue so basically where to start, unless you told them not to? even then the shop i take my stuff to probably would advise against it unless i told them why i know its XXX issue

Farmers Feed the World said "We have a 2012 rav 4 and are having an issue with the heater fan. It will only run on number 4 setting (max). I've had issues like this in the past with other vehicles and its the relay. I had Toyota change the relay but the problem came back. I'm leaning towards a bad spot on the blower motor, Toyota thinks it could be the selector switch. I want to trouble shoot the problem before Toyota gets a hold of it again nothing is covered under warranty so I want to know what I need here. Has anyone else had an issue like this? Discouraging I'm needing to repair a 3 year old vehicle. "

If the fan speed resistor was changed once already and the problem has returned, it's possible the fan motor is drawing too much current and that's what keeps toasting the resistor.

I'm more experienced with domestic vehicles. But if replacing the resistor didn't work, I've ended up changing the selector switch after which solved the problem.