2000 Honda CRV Temperature Gauge Rises at Idle?

When my Mom comes to a stop sign on a hill near our house, the temperature gauge shoots up into H. If she turns the heater on the gauge drops back down to about halfway. We replaced the thermostat yesterday and added coolant to both the radiator and overflow. She drove the car today and stopped at the same Stop sign. Temperature is still shooting up. Checked overflow and radiator. Both still have adequate coolant. Need help determining cause of temperature issue. It only happens when the car is sitting at idle on a hill.

Any help or recommendations would be greatly appreciated. We want to avoid seeing an mechanic.

Sounds like the engine fan isn't running.

Check to see if the fan is operating when it's hot.

All cars will get hot if stationary with engine running
(being on a hill should make no difference - so try doing the same on the flat)
car has an electric fan that cuts in when temperature gets high (you should be able to hear it cut in)

If fan not coming on when it gets hot could be fan not working or fan temperature sensor not working
Check fan by connecting its lead across battery

If fan ok then could be water pump or could be airlock in cooling system

Are the fans starting when needed?
Does putting the AC on stop it?

Only four things more to check:

1. Malfunctioning electric fan
2. Blocked or clogged radiator
3. Broken fan belt
4. Failing water pump

When ti get past half way get out and check if the fan is on if not that is your problem.

Classic symptoms or a failing or failed cooling fan.

In my experience it sounds like a radiator fan or a stuck thermostat