Heater core keeps going bad?

Good morning,

Using a voltmeter, please check for voltage in the coolant. Take your red lead and put it in the coolant and the black lead to good chassis ground. Please give me the voltage.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

Check your pressure cap as well. If there is too much pressure, it will fail the heater core.

https://www.2carpros.com/articles/radiator-pressure-test

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

https://www.2carpros.com/articles/replace-heater-core

I attached the procedure below for you as well.

This could be from the bad ground but the test in the coolant will tell.

Please keep me updated with the results.

Roy