Tankless water heater shows E3 and shuts down; unit may start but quickly stops with no hot water.

Safety first: Turn OFF power and gas. Hot exhaust components can cause burns; allow the unit to cool.

• Step 1 – Inspect vent termination: Go outside and check the Ruud vent termination for obstructions (bird nests, insect screens clogged, snow, leaves). Clear any blockage.
• Step 2 – Check venting configuration: Compare your vent length, elbows, and pipe size to the Ruud installation manual. Excessive length or too many elbows can cause high exhaust temperature. Correct any obvious installation errors.
• Step 3 – Inspect exhaust temperature sensor: Remove the front cover and locate the flue/exhaust thermistor on the exhaust collar. Check for loose wiring or corrosion. Reseat the connector.
• Step 4 – Reset and test: Restore power and gas, then run hot water. If E3 returns immediately, the exhaust thermistor may be faulty.
• Step 5 – Replace exhaust thermistor: With power and gas OFF, remove the sensor from the exhaust collar, disconnect wiring, and install a new OEM Ruud exhaust temperature sensor. Reassemble and test.

If venting appears improperly installed or modified, contact a qualified technician to redesign the vent system to Ruud specifications.