Tankless water heater shows E5 and shuts down; water temperature may fluctuate before shutdown.

Safety first: Turn OFF power and close the cold water supply valve to the heater. Allow the unit to cool.

• Step 1 – Clean inlet filter: Locate the cold water inlet filter at the bottom of the unit. Place a towel or pan under it, unscrew the filter, and rinse out debris. Reinstall and open the water valve.
• Step 2 – Check minimum flow: Ensure at least one hot water faucet is fully open when testing. Very low flow may not activate the sensor.
• Step 3 – Inspect flow sensor: Remove the front cover and locate the water flow sensor on the cold inlet piping. Check for loose wiring or visible damage. Reseat the connector.
• Step 4 – Descale if needed: If the system is heavily scaled, perform a full descaling of the heat exchanger and piping using a pump and descaling solution through the service valves.
• Step 5 – Replace flow sensor: Turn OFF water and relieve pressure by opening a hot faucet. Remove the screws or clips holding the flow sensor, disconnect wiring, and install a new OEM Ruud flow sensor. Reassemble and test.

If E5 persists after sensor replacement, the control board flow circuit may require professional diagnosis.