2018 RT27-OB water heater problem

My first request for help, so I hope I am doing it right. HIN number FMLT2744L718. The ISO temp heater, which runs off 120 electric only, will only get the water luke warm. The mixing valve has been set from full on to full off with a one hour wait each time to see if there was a difference in the setting. There was not. The breaker is on with a green light showing power to the circuit. The cover was removed and the reset button was pressed. One hour later there was still no change in the temp.

The possibilities appear to be a stuck mixing valve, a bad heating element?,a bad service thermostat, or a bad safety thermostat. Is there a way to troubleshoot those parts with a multi-meter so I don't drag this repair out for weeks?

Thanks,
John Cortesy

OK here is a follow up. Troubleshooting so far:

Checked for voltage on the incoming lines. 119 volts present.

Checked over-temp safety thermostat red button not popped. Button was in down position.

Checked for pass through voltage on other side of safety thermostat. 119 volts present on other side.

Checked impedance of main thermostat. Impedance at zero indicating a normally closed position.

Rotated thermostat setting knob counter-clockwise. Impedance went to infinite.

Rotated knob back to original setting. Impedance returned to zero.

Checked impedance across the two sides of heating loop. Impedance at 17.2 Ohms.

Here is the math.

• R= (119V) squared/750
• R= 14161/750
• R= 18.8 Ohms

Is the difference in impedance, 1.6 Ohms, enough to make that big of a temperature difference? The output water cannot be more than 75-80 degrees.
Should I replace the Heating coil?
Is the Thermostat reading correctly?
Should I replace that?
Any help would be appreciated.

Thanks,

John Cortesy