Safety First: Turn off the breaker before opening any panels.

Check breaker and disconnect: Verify the pool heater breaker is ON and any external disconnect is closed. Reset if tripped; if it trips again, stop and call a professional.
Inspect power wiring: Remove the electrical access cover and confirm that line wires are securely attached and not burned or loose.
Check internal fuse: Many Rheem heaters have a low-voltage fuse on the control board. With power off, remove and visually inspect or test with a meter. Replace only with the same type and rating.
Test transformer output: If equipped, measure low-voltage output (e.g., 24 VAC) from the transformer. If there is no output but proper line voltage in, replace the transformer.
Control board: If power and low-voltage are present but the display is blank, the main control board may be defective and require replacement.

Heater will not start at all; no display, no lights, completely dead

Possible Causes