Dear Ask a Mechanic,
I have a 2009 Chevy Cobalt that I use mostly for commuting from home to the local GO station. Right now, the heater fan only works on high speed, and it’s annoying. I’ve replaced the resistor for the speeds once already, and it’s failed again. It hasn’t even been two years. Why does it go bad, and can I do anything to prevent it? – Fan Frazzled
There are several different types of blower resistors used in vehicles, but they control fan speeds in one of two ways; either what’s known as pulse-width modulation or through resistance.
Pulse-width modulation is a fancy way of saying that the circuit is switched on and off quickly, reducing its average voltage. The longer it is “on” relative to “off,” the higher that average. This ratio is known as the duty cycle. While it once was only used for fans in automatic HVAC systems, it’s now common in manual systems, too. Electronic speed controls for blower fans typically have a sizeable aluminum heat-sink to keep them cool.
The second way of providing fan speeds is resistance, and, as you note, that is what’s used in your Cobalt. Every speed has its own circuit from the switch, and all but high speed goes to – and through – resistors mounted to a ceramic board that’s cooled by air from the blower. Each resistor reduces the voltage available to the fan by a different amount. (Older designs used metal coils for the same purpose.) Fortunately for you, this style is usually less expensive to replace.
There are several things that tend to kill blower resistors. Corrosion, from condensation or water leaks, and overheating because of restricted airflow due to a blocked cabin filter or debris ingested into the system, are the two that I commonly see.
Probably the most common reason though is excessive current being drawn from the fan motor. If the fan still works on high speed and isn’t noisy or slow, the issue won’t be obvious. Worn bushings and brushes can add enough current draw to overload the resistor.
If your resistor doesn’t have readily apparent corrosion damage, I’d suspect the blower motor is responsible for the failure. Note that Cobalts are oddballs in that you must cut the old motor out of its casing. The new one will bolt in place.
Bear in mind that while resistors are the most common failure in scenarios like yours, switches and connectors can also cause the same problems. A switch that feels odd may be damaged internally and any melted connectors will be obvious when they are inspected.
Ask a Mechanic is written by Brian Early, a Red Seal-certified automotive technician. You can send your questions to email@example.com. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.
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