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My Pontiac Montana is blowing warm air with the heater turned on. I was receiving a code(not sure what the number was but the message referred the engine was running cooler than it was supposed to.) Since it never got over 140, I thought it was a stuck thermostat. Replaced it with a 195 thermostat. Now it gets up to 180, cools and goes back to 140. The heater heats up after it opens but cools down quickly after. It only is hot for about 15 seconds, followed by 30 seconds of lukewarm.

Ideas why the heater never heats up?

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I'd vote heater core issue first. If the heater core is clogged you might not be getting enough hot coolant through to get significant heat after the coolant cools down to a normal temp.

The other option on some cars is that the thermostat control itself isn't working properly this would be more likely if the car has a digital thermostat and the control is on the fritz.

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