Q: heater doesn't always work on 2001 Toyota Celica

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The heater doesn't always work. Sometimes it will come on after driving for a while. I have checked the water level and the leaving water line out of the heater is cold. What could it be?
(1) Answer
Does the engine warm up completely? I'm wondering about the coolant thermostat being defective and allowing the engine to run too cold. Another thing is that there may be a problem in the heater control module in the dash or that the blend air door actuator is not working. It's also possible that the heater core is plugged or there's a blockage in the lines to or from the heater.
You may need a Toyota specialist for some help on this.
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