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Q:

no heat all of a sudden only blows cold on 2006 Chevrolet Trailblazer SS

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(1) Answer
Check for the temperature of the coolant hoses going into and out of the heater core at the back of the engine compartment. If they are scalding hot, then you have a temperature control issue inside the truck. If they are cool or cold (or one is) then you may have an engine thermostat issue.