1999 Mercury Sable v6, 143,000 miles. No hot air for over a year. While driving, the temperature needle goes nearly to the top for about 30 seconds then back to the normal position. Usually happens while driving on an incline.
Could be a slow opening theromostat. Also, remove the radiator cap, and check the coolant level.
The temperature gauge is working normally. You need to see an independent mechanic to get the more serious problem corrected. No heat could be caused by several different problems. The cooling system must work properly to keep your engine from overheating.
Coolant is fine. Water pump about 2,000 miles ago, new plugs and wires. Mechanic said I had a blown head gasket, but there are no other “symptoms” to verify that.
With a 143,000 miles on the vehicle, the no heat problem could be caused from a plugged heater core. And with the overheating problem when the engine is put under load such as climbing a hill, the radiator could be plugged up where it’s no longer passing enough coolant to keep the engine cool under that operating condition.
feel the heater hoses. If it’s plugged, both should be somewhat cold.
The climbing temp, assuming it’s a sustained hill climb, might be nothing serious. Get the coolant tested for combustion dissolved gases.