Status LED flashes code 21 (two short, one long). Furnace may not respond to thermostat or may behave erratically.

Safety first: Turn off power before accessing the control board. Avoid touching live terminals.

• Reset power: Turn off furnace power for at least 1 minute, then restore. Clear any thermostat call and re-initiate a call for heat. Observe if the code returns.
• Inspect thermostat wiring: With power off, check for bare spots or shorts in the thermostat cable between R and W. Ensure no wires are pinched by the furnace cabinet.
• Check for stored fault history: Some Payne boards use 21 to indicate a previous lockout. Clear the fault by cycling power and observe the next operating cycle for more specific codes (e.g., 14, 34).
• Verify 24 VAC transformer output: Measure between R and C on the control board; you should see about 24 VAC. If low or unstable, the transformer or board may be failing.
• If code persists with no other obvious cause: The integrated furnace control board may be defective and require replacement. Label all wires and take photos before swapping the board.