E18
Alpha
Gas Valve Modulation / Stage Control Fault
Furnace displays E18; two-stage or modulating models may be stuck in low fire or fail to change stages.
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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
Failed two-stage gas valve coil, Faulty modulation signal from Alpha control board, Loose staging harness, Incorrect thermostat configuration for staging
How to Fix / Troubleshooting
Safety: Turn off power and gas before touching the gas valve or wiring.
- Verify thermostat settings: Ensure the thermostat is configured for 2-stage or modulating heat if your Alpha furnace supports it. Incorrect settings can cause staging errors.
- Inspect staging wires: Check W1/W2 or modulation signal wires between thermostat and control board for secure connections.
- Check gas valve harness: Confirm the multi-pin connector on the Alpha two-stage or modulating gas valve is fully seated and undamaged.
Testing staging signals and gas pressures should be done by a qualified technician. Do not attempt to manually force stages at the gas valve.
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Repair Difficulty
Professional Required
5/5
Required Part
Alpha Two-Stage Gas Valve
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