Heater locks out with E03; unit may start then quickly shut down, or may not start at all.

Safety first: Turn off power to the heater and stop the circulation pump. Allow the heater to cool before touching any internal components.

Step-by-step checks:

• Verify water flow: Ensure the pool circulation pump is running and that valves before and after the Pahlen heater are fully open. Check the filter pressure; a very dirty filter can reduce flow and cause overheating. Backwash or clean the filter as needed.
• Check bypass and flow settings: If your system uses a Pahlen automatic bypass or manual bypass valve, confirm it is adjusted according to the installation manual. Too little flow through the heater can cause rapid temperature rise and high limit trips.
• Inspect inlet/outlet for restrictions: Remove unions at the heater inlet and outlet and check for debris, scale, or collapsed gaskets obstructing flow.
• Reset high limit (if manual reset type): Locate the high limit thermostat on the heat exchanger header. Some Pahlen models have a small red reset button. Once the heater is cool, press to reset. If it trips again quickly, do not continue resetting without finding the cause.
• Check for scale in heat exchanger: If the pool water has been hard or chemically imbalanced, the copper or stainless Pahlen heat exchanger may be scaled. This often requires chemical descaling or replacement by a professional.

When to stop: Repeated E03 trips after confirming good flow indicate a serious overheating or scaling issue. Contact a qualified technician to inspect the heat exchanger and safety controls.