"Saab thermostats are special. They have a three position operation. the first position is full closed and it blocks all flow to the radiator until the engine warms up. The second position allows flow both to the heater core and the radiator to provide cabin heat and cool the engine. This is the position the thermostat should be in most of the time while driving. The third position of the thermostat allows full flow to the radiator and blocks flow to the heater core as the plate on the bottom of the thermostat blocks the passage to the pipe to the heater. The thermostat reaches this position when the engine is nearing overheating and allows coolant to flow directly to the most efficient heat exchanger, the radiator. The heater core is bypassed because the heater valve has bypass circuits in it and is not exposed nor has the flow capabilities of the radiator. This allows for maximum cooling of the engine when it is needed. Aftermarket designs usually do not incorporate these three positions, only two. The biggest complaint from people who use incorrect aftermarket thermostats is no heat after the car warms up in the winter."
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