43
Heil
Secondary Heat Stage Fault
Heil furnace LED shows code 43; furnace may not stage correctly or may not reach full capacity.
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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
Open W2 call from thermostat, Loose W2 wiring, Faulty gas valve second stage coil, Control board staging fault
How to Fix / Troubleshooting
Safety: Turn off power before adjusting thermostat or control wiring.
Steps to check:
- Thermostat settings: Ensure your thermostat is configured for 2-stage heat if your Heil furnace is 2-stage. Check installer setup menu.
- Check W2 wiring: Verify that the W2 terminal at the thermostat is connected to W2 at the furnace control board.
- Inspect gas valve wiring: For 2-stage gas valves, ensure both stage coils are wired correctly and connectors are tight.
- Test operation: Raise the thermostat several degrees above room temperature to force second stage and observe if the furnace ramps up.
When to call a pro: If second stage never engages, a technician should test W2 signal, gas valve coils, and control board staging logic.
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Repair Difficulty
Medium
3/5
Required Part
Two-Stage Gas Valve
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