Heater locks out with H4 error; fan may run briefly then stop (on forced-draft models).
Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
Possible Causes
How to Fix / Troubleshooting
Safety first: Turn off power to the heater. Do not operate the heater with a suspected flue blockage; carbon monoxide risk is serious.
- Inspect flue terminal: Check the AstralPool flue cowl or terminal for bird nests, leaves, or obstructions. Clear any debris.
- Check combustion fan operation: With the front panel removed and power restored, call for heat and observe whether the fan starts. If the fan does not run, turn power off and check its wiring harness and connector.
- Inspect pressure switch tubing: Locate the silicone tube between the fan housing and the air pressure switch. Ensure it is not cracked, kinked, or disconnected. Replace if brittle.
- Test pressure switch (basic): With power off, gently tap the switch body; sometimes a stuck diaphragm can free up. Do not bypass the switch for operation.
If H4 persists, the air pressure switch or fan motor may need replacement and should be tested by a qualified technician.
Repair Difficulty
Required Part
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