Display shows E04 and heater will not fire; may occur immediately on a call for heat.

Safety first: Turn off power to the heater and shut down the circulation pump before opening any plumbing connections.

Step-by-step checks:

• Confirm pump operation: Make sure the pool pump is running at the correct speed (for variable-speed pumps) and that skimmer baskets and pump strainer are clean.
• Check filter and valves: Clean or backwash the filter. Verify that all isolation valves around the Pahlen heater are fully open and that any bypass valve is not diverting too much flow away from the heater.
• Inspect flow switch orientation: Locate the Pahlen flow switch or flow sensor body in the inlet line. Confirm the arrow on the body matches the direction of water flow and that the switch is installed in a straight run of pipe as specified.
• Clean or replace flow switch: Remove the flow switch from the tee or housing. Inspect the paddle or turbine for scale, debris, or damage. Clean gently and ensure it moves freely. If the paddle is broken or heavily scaled, replace the flow switch.
• Check wiring: Inspect the cable from the flow switch to the control board for cuts, abrasion, or loose connectors. Reseat the plug on the control board.

When to stop: If E04 persists despite confirmed good flow and a clean/new flow switch, the control board may be misreading the sensor and should be evaluated by a technician.