Since you have a multimeter, first test whether the heating element is in good condition, and then test whether the thermostat contact can be opened and closed normally.

Because the interior is burnt out by water, it is very important to check whether the metal sheet in the base is deformed or ablated, and it can usually be restored to normal use after adjustment and repair.

If it still doesn't work, it depends on other components on the circuit board. For example, the components under the radiator should be removed and measured with a multimeter.

There is also whether the rectifier and filter circuit components are burnt out, which affects the normal power supply of DC.