My jimmy temp gauge is staying over 210 and now the heater is not getting hot.

1. Check the coolant level in the tank, and top up if necessary.
2. Inspect the cooling system for leaks and repair if necessary.
3. Verify the cooling system if the coolant flows correctly inside radiator, pipes & hoses, and heater core, engine idling. Check the thermostat also. See the coolant colour. Flush the cooling system and change the engine coolant if the coolant is too old.
4. Check up the water pump belt and replace if necessary.
5. If the belt is all right, replace the water pump. The coolant MUST flows correctly inside cooling system and heater core. Check in special the input and output heater core pipes temperature. Both of them MUST be hot, engine idling.
6. Verify the heater module inside car, if the warm temperature (air mixing) flap works correctly in "HOT" position. Repair the flap if necessary.
7. If everything above mentioned were done, then you need to remove the heater core and clean inside or change it.