21
Keeprite
Gas Heating Lockout or Control Fault (Model-Dependent)
Furnace LED shows code 21; unit may not start or may behave erratically.
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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
Repeated ignition failures, Faulty control board logic, Incorrect line voltage or polarity, Loose control board connections
How to Fix / Troubleshooting
Safety: Turn off power and gas before inspecting wiring.
Steps:
- Reset power: Turn off the furnace service switch for 30 seconds, then turn it back on to clear a temporary lockout.
- Check line voltage and polarity: Verify that hot and neutral are correctly wired to the Keeprite control board L1 and neutral terminals. Incorrect polarity can cause control faults.
- Inspect board connections: Ensure all multi-pin plugs and spade connectors on the control board are fully seated and not corroded or burnt.
- Review other codes: If code 21 appears after repeated ignition attempts, troubleshoot ignition-related codes (e.g., 34, 14) first.
If code 21 persists with proper wiring and gas supply, the control board may be defective and should be tested and replaced by a qualified technician.
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Repair Difficulty
Hard
4/5
Required Part
Furnace Control Board
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