Dear Car Talk:
I have a 2005 Chrysler 300C with about 95,000 miles and automatic temperature control. On hot days, on trips of 30-45 minutes or more, the fan speed gradually slows down, and if the trip is long enough, the fan will pretty much cease to move any air at all. I've had the fan resistor replaced twice, and I believe that it is failing again. But before I take it in again, I'm wondering if something else could be causing the problem.
I think something else is causing the problem, Steve; it doesn't sound like the resistor.
Usually, if the resistor burns out, you get either one speed (high), or no speeds (all of the other settings). The resistor is what allows the fan switch to give you three or four different speeds, by varying the amount of power that gets sent to the fan motor. If it's just slowing down over time, it's much more likely that your fan motor is dying.
I'm surprised it hasn't blown the fuse yet. Usually, if the fan motor is dragging, that'll create a lot of resistance. Then the current draw will go way up and the fuse will blow. That may be the next thing you notice, Steve.
A less-likely possibility is that a faulty blend door is slowly closing and not allowing the air to blow into the passenger compartment. But a fan motor would be my first guess.
Ask your shop to put an ammeter in the circuit and see how many amps the fan is drawing. I'll bet if it has a 20-amp fuse, it's drawing about 19.8 amps. Good luck, Steve.