1993 GMC Sierra 5.7L Std Cab Pickup Heater not blowing warm air. Thermostat has been replace as well as coolant top-off
Check the water control valve behind the engine close to the firewall. This valve opens and closes to allow hot coolant to enter the heater core. It is usually connected to the heater controls with a cable. When you adjust the heater control for heat, a cable activated lever on the valve should move and allow coolant to move through the valve. The cable from the heater control can come loose from the lever, or the valve itself can become corroded and hard to move.
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