Heater displays E04 and will not fire or shuts down quickly after starting.

Safety: Turn off power and allow the heater to cool completely before opening panels.

• Check water flow first: Confirm the pump is running, filter is clean, and valves are open. Low flow is the most common cause of high limit trips.
• Inspect high limit switches: Pentair heaters typically have one or more high limit switches mounted on the header. Check wiring for loose or corroded connections. With power off, test continuity across each switch at normal temperature. Replace any switch that is open at ambient temperature.
• Look for scale or blockage: If high limits trip repeatedly, the heat exchanger may be scaled or partially blocked, causing localized overheating. This requires disassembly and descaling by a qualified technician.
• Reset and monitor: After addressing flow and connections, restore power and run the heater while monitoring temperature and flow. If E04 recurs quickly, stop using the heater and schedule professional service to prevent damage.