Rheem tankless water heater displays E0 and will not run a hot water cycle

Safety first: Turn OFF power at the breaker and close the gas shutoff valve (for gas models) before removing any covers.

• Power reset: Turn the unit OFF at the breaker for 5 minutes, then restore power and see if E0 clears.
• Inspect control cable: Remove the front cover. Locate the low-voltage harness between the user interface (front keypad/display) and the main control board. Ensure both ends are fully seated and not corroded or pinched.
• Check for moisture: Look for signs of water drips or condensation on the control panel and main board. Dry gently with a hair dryer on cool setting and correct any leaks above the unit before re-energizing.
• Test with minimal connections: Disconnect any optional remote controllers or accessories and try running the heater with only the main control panel connected.
• If E0 persists: The user interface PCB or main control board is likely defective. Replacement typically requires removing multiple connectors; label each connector and take photos before removal. If you are not comfortable working with electronics, contact a qualified technician.