Rheem tankless water heater displays E4 and water temperature fluctuates or unit shuts off

Safety first: Turn OFF power and gas. Do not operate the heater with a suspected venting problem.

• Inspect venting: Check the intake and exhaust terminations outside for obstructions (leaves, nests, snow, louvers). Ensure vent pipes are not crushed or disconnected.
• Check clearances: Make sure nothing is blocking the air intake around the unit.
• Locate flue thermistor: Remove the front cover and find the sensor mounted on or near the exhaust collar or combustion chamber.
• Verify wiring: Ensure the thermistor connector is fully seated on the control board and wires are intact.
• Test sensor (advanced): Measure resistance and compare to Rheem's chart. Replace if out of range.
• If venting is restricted or improperly installed: Correct vent length, slope, and fittings per Rheem installation manual. Venting errors can cause dangerous exhaust conditions; consult a licensed gas technician if unsure.