Safety: Turn OFF power and allow the unit to cool completely before servicing.

Corrective actions:

Remove the front cover and locate the high-limit thermostat on the heat exchanger.
Press the manual reset button if present. If it will not reset or immediately trips again, do not force it.
Check the inlet filter and any external strainers for blockage. Clean thoroughly.
Inspect the heat exchanger and elements for scale. Perform a full descaling procedure if buildup is present.
Verify that the minimum flow rate for the model is being met. Very low flow can cause overheating; adjust fixtures or flow restrictors as needed.
If the high-limit thermostat does not reset or appears damaged, replace it with the correct Eemax high-limit thermostat.
Reassemble, restore water and power, and test operation while monitoring outlet temperature.

If E9 returns after these steps, the control board may be mismanaging element firing and should be evaluated by a technician.

Display shows E9; unit may shut down during operation, especially at high temperatures.

Possible Causes