I had a 2001 Suburban that the heater did not blow hot air. Sometimes the coolant temperature would show 180 F; sometimes it would show 205F. I couldn't figure out why the coolant temperature would go cool, even when traveling at highway speeds. I removed the thermostat and found parts of an o-ring seal had migrated into the inner spring of the thermostat. Sometimes this debris would prevent the stat from fully closing. This would cause the coolant temperature to be too cool for the heater to blow hot air. Replaced the thermostat; problem solved. Coolant temp steady at 205F and heater blowing hot air.