Poolex pool heater shows F4 and will not start heating.

Safety: Isolate power to the Poolex unit before removing panels or touching wiring.

Diagnostic steps:

• Power cycle: Turn the heater off at the breaker for 2–3 minutes, then restore power to see if F4 clears (temporary logic glitch).
• Locate inlet sensor: The water inlet NTC sensor is typically clipped or screwed into the inlet pipe or titanium heat exchanger manifold. Consult your Poolex model diagram.
• Inspect wiring: Check the sensor cable for cuts, rodent damage, or pulled connectors. Reseat the plug on the main control board.
• Check for corrosion: Look for green/white corrosion on the sensor connector and board terminals. Clean lightly with contact cleaner if accessible.

Replacement (for competent DIYers):

• Order a compatible Poolex water inlet NTC sensor for your exact model.
• Turn off water flow and relieve pressure if the sensor is in direct contact with water.
• Remove the old sensor, install the new one with any supplied O-ring or thermal paste, and reconnect the wiring.
• Restore power and verify that F4 is cleared and temperatures read correctly.

If you are not comfortable working with electrical components, have a technician test the sensor resistance and replace it if out of spec.