Check into the temperature control valve--this is different than the thermostat.
How did you change the thermostat?
it could be that you've put too much coolant to percentage of water in
Check the thermostat or heater core
I suggest replacing the thermostat, that might work.
Sounds to me that the thermostat is in backwards.
May be a defective thermostat or heater core
Could be a bad thermostat.
Insufficient amount of coolant or thermostat not opening
probably the heater core is clogged
replace anti-freeze, replace thermostat
Because your thermostat is probably stuck open
To test a hot water heater thermostat, you can use a multimeter to check for continuity in the thermostat. First, turn off the power to the water heater. Then, remove the access panel to expose the thermostat. Use the multimeter to test the thermostat's terminals for continuity. If there is no continuity, the thermostat may be faulty and needs to be replaced.