What It Means
Gas heating lockout due to miswired gas valve or defective relay.
Carrier Furnace
Carrier Furnace Error Code 21
Troubleshooting guide for the Carrier Furnace system fault 21.
Possible Causes
- Incorrectly wired gas valve harness.
- Defective gas valve solenoid coil.
- Stuck or damaged control board relay.
Not sure which one it is — or if it's none of these? Try ACLogics today.
Disclaimer: This information is for general reference. Error codes can vary by model and production year. Always refer to your machine's specific manual and the information printed on the unit itself.
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Full Diagnosis
Gas Heating Lockout. Control will not auto reset; check mis-wired gas valve or defective control relay.
All Recorded Causes
Inspect gas valve wiring for incorrect connections. Test gas valve coil resistance. Check control board relay output voltage during call for heat. Replace gas valve or control board as needed.
Verify Before Replacing Parts
- Confirm the exact Carrier model family and furnaces context before acting on a generic code description.
- Walk the entire heating sequence in order instead of jumping straight to the reported code, watching for inspect gas valve wiring for incorrect connections.
- Verify each safety device and prove where the sequence stops under live operation, including whether test gas valve coil resistance.
- After the correction, run the system long enough to confirm the furnaces fault does not return under normal load.
Still unsure about this fault?
ACLogics walks you through Carrier Furnace diagnostics step-by-step — so you confirm the cause before swapping parts.
