21
Moderate
Carrier · Central Air Conditioners & Furnaces
Gas Heating Lockout
Quick Answer
Error code 21 on your Carrier Central Air Conditioners & Furnaces means: Gas Heating Lockout. This is usually fixable — see the step-by-step guide below.
Likely Causes
How to Fix It
1
Verify gas supply is on and other gas appliances in the home are working
2
Check gas pressure at the inlet and outlet of the gas valve using a manometer
3
Test gas valve coil resistance with a multimeter (typically 150-600 ohms)
4
Verify 24VAC signal is being sent to gas valve from control board during ignition sequence
5
This repair involves gas line work and should be performed by a licensed HVAC technician
Pro Tip
This fix requires some technical knowledge. If you're unsure about any step, consider contacting support or a qualified technician.
Parts You May Need
Gas valve assembly
Hot surface igniter
Flame sensor
Gas valve wiring harness
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