Heater shuts down with E04; may start briefly then stop, often related to high limit temperature.

Safety: Turn off power and allow the heater to cool before touching internal components.

What to check:

• Water flow: Confirm strong return flow to the pool. Clean filter, verify valves are open, and ensure bypass valves are correctly set so adequate flow passes through the heater.
• Air in system: If you recently serviced plumbing, bleed air from high points and ensure the pump is fully primed. Air pockets can cause hot spots in the heat exchanger.
• Inspect high limit switches: Lochinvar pool heaters typically have one or more high limit switches mounted on the water header. Check wiring connections for corrosion or looseness. Lightly wiggle connectors to ensure good contact.
• Check for scale: If the heater is older or used with hard water, internal scale can restrict flow and cause overheating. Signs include noisy operation and frequent high-limit trips. Chemical descaling of the heat exchanger should be performed by a professional.
• Reset and test: Some models have manual-reset high limit switches (small red button). After cooling, press the reset button if present. If the switch trips again quickly, do not continue resetting—address the underlying cause.

Persistent E04 after confirming flow usually indicates a failing high limit switch or a scaled heat exchanger that requires professional service.