The Chevrolet design is much less susceptible to failure than the Fords and Chryslers, but they are not immune. Two things help the Chevy.

1. The actuator motor has built-in limits and the motor is capable of turning only ~160 degrees. The motor cannot continue to spin and cause an immediate breakage. While the motor moves 160 degrees, the doors only have about 120 degrees of movement, so the motor does apply excessive force after the door closes, but it is limited by the internal motor stops. The door will "bend" but the motor stops before it breaks.

2. Chevy plastic blend doors are better designed and are stronger than Ford or Chrysler. The plastic will still break, but since it's stronger, it takes longer.

The same basic fail mode that we address on other automobiles exists on the Chevy, but because of the motor and plastic, it takes a lot longer to fail and we don't see the same immediate market as the other vehicles. We have prioritized other product development over Chevrolet because the market hasn't really developed yet.

All that said, your '99 could easily be on the leading edge of what will eventually become a common failure. A few things to check:

1. The heater hoses will get hot whether coolant is circulating or not. Try feeling the hoses from a cold start and check that they warm up at about the same rate and get uncomfortably hot at about the same rate as the radiator does. You can always disconnect the heater hoses and flush with a garden hose to be sure. On a vehicle this age, it wouldn't be a bad idea to do a good radiator flush anyway. You'll get better cooling efficiency and clean out the system. Prestone makes a good DIY kit that you can find at Wallmart.

2. You can remove the actuator motor and get to the blend door axle directly. You should be able to turn the door with your fingers and switch between heat and no heat manually. If the door feels spongy or you see cracks in the connector, the door is the problem.

3. We have also seen problems with the re-circ door on the Blazer and it has the same electronic control as the blend door. If this is broken or leaking, it will allow external air into the system and can significantly affect heating efficiency, but won't kill heat completely. Check FAQ #2 for some simple diagnostics for the re-circ door. It's written for the Jeep GC, but the operation is the same on your Blazer.


The heater system is fairly simple. If air is blowing out of the fan, the heater core is hot, and the blend door is directing air through the core...you have heat. It has to be one of those three elements that is causing any problem.

We would greatly appreciate any feedback on what you find and of course, digital photos are gold to us. We are in the mode of assessing the Chevy market and any information would be great.