AFS
Laars Air Flow Switch / Proving Switch Fault

Boiler displays AFS or Air Flow Switch Fault; unit will not fire.

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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.

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Possible Causes

Stuck closed or open air flow switch, Incorrect tubing connection to switch, Faulty switch contacts, Inducer not providing correct pressure

How to Fix / Troubleshooting

Safety first: Turn off power before working on the air flow switch or tubing. Do not bypass the switch.

  • Inspect tubing: Verify that the small tubes from the inducer or venturi to the air flow switch are connected to the correct ports and are not cracked, kinked, or filled with water.
  • Check for water in tubing: Remove the tubing and blow through it. If water is present, dry or replace the tubing and check for condensate issues.
  • Tap the switch lightly: With power off, gently tap the switch body to free a stuck diaphragm. Do not hit hard.
  • Test operation: Restore power and call for heat. If AFS persists, the switch or inducer may be faulty.

When to call a professional: Replacement of the air flow switch and verification of inducer performance should be done by a technician.

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Repair Difficulty

Medium 3/5

Required Part

Air Flow / Pressure Switch
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