Check/Repair
Turn the power off to the water heater and remove the control box cover. Using a multi-meter, check the ohm resistance of the upper and lower heating elements between the blue and yellow wire connections (upper element) and the orange and blue wire connections (lower element). The ohm resistance should measure between 5-25 ohms. Replace the element if it does not measure 5-25 ohms., If the heating elements are okay, remove the thermistor plug from the control board. Using a multi-meter, measure the resistance between the red wires in the thermistor plug, then measure the resistance between the two black wires in the plug. Open a hot water faucet and measure the water temperature. If the water temperature is between 80 and 100 degrees Fahrenheit, the thermistor should measure between 50k-20k ohms; 100-120 degrees should measure 40k-15k ohms; 120-150 degrees should measure 30k-5k ohms. If the thermistors and heating elements are within ohm specification, replace the control board.