Heater shuts down with E05; may start and then quickly stop

Safety First: Turn off power and gas. Water in the heater can be very hot; allow time to cool.

• Check water flow: Ensure the pump is running at the correct speed and that the filter is clean. Verify all valves around the heater are fully open.
• Inspect bypass settings: If your Rheem heater uses an internal or external bypass, confirm it is adjusted per the installation manual. Too much bypass can cause localized overheating.
• Examine high limit switches: Locate the high limit switches on the header. With power off, check wiring connections. If the switches are frequently opening, they may be weak or the exchanger may be scaled.
• Descale heat exchanger (pro recommended): Heavy scale buildup restricts flow and causes hot spots. A professional can chemically descale the copper or cupronickel heat exchanger following Rheem procedures.
• Replace faulty high limit: If testing shows a high limit switch is open at normal temperatures, replace it with a Rheem-specified high limit switch.