Rheem tankless water heater displays E2 and burner will not ignite

Safety first: Turn OFF power and gas. Allow the unit to cool completely.

• Locate inlet thermistor: Remove the front cover. Find the thermistor screwed into the cold water inlet pipe or manifold (often labeled TH1 or Inlet).
• Check wiring: Ensure the thermistor plug is fully seated on the main control board and that the wires are not pinched or cut.
• Inspect sensor: Look for corrosion, mineral buildup, or physical damage at the sensor body. Clean light scale with a soft brush.
• Resistance check (advanced): With power OFF and sensor disconnected, measure resistance with a multimeter. Compare to Rheem service chart (typically ~10kΩ at 77°F/25°C). If reading is open, shorted, or far out of range, replace the thermistor.
• Replace if needed: Shut off water, relieve pressure, unscrew the old thermistor, install the new one with appropriate sealing (factory O-ring or thread sealant as specified), reconnect wiring, and restore power.