Display shows E5; unit may heat erratically or shut down during operation.

Safety: Turn OFF power at the breaker and close the water supply before opening the heater.

Diagnostic steps:

• Remove the front cover and locate both inlet and outlet thermistors.
• Inspect wiring harnesses for both sensors. Ensure connectors are fully seated and wires are not pinched or damaged.
• With power OFF, disconnect each thermistor and measure resistance with a multimeter. Compare both readings at the same water temperature; they should be similar and within Eemax spec.
• If one sensor is significantly off or open/shorted, replace that thermistor sensor with an OEM part.
• Inspect the heat exchanger and elements for scale buildup that could cause uneven heating. Descale the unit if necessary.
• Check that the unit is not oversized for the flow rate; extremely low flow can cause large temperature jumps. Adjust flow restrictors or temperature setpoint as needed.
• Reassemble, restore water and power, and test operation. Monitor the display temperature and verify E5 does not reoccur.

If both sensors test good and the unit is clean, the control board may be miscalculating temperature differential and should be replaced by a technician.