HVACopedia

Carrier · Furnace

21.1 24 VAC on gas valve when it should be off

Critical severity 59SC6A

What the control is telling you

The control detects 24 VAC at the gas valve circuit when no gas valve output should be present.

Applies to: Carrier 59SC6A

Likely causes

  • Gas valve relay on control stuck or failed
  • Miswired or shorted gas valve circuit

Test and repair steps

  1. Warning: Shut off gas to the furnace immediately if burners stay lit with no call for heat.
  2. Remove the thermostat call and measure voltage at the gas valve terminals.
  3. Inspect gas valve wiring for shorts or miswiring.
  4. If 24 VAC remains on the valve with no call and wiring is correct, replace the furnace control.
  5. Program any new control with the Model Program Number on the rating plate (app via NFC or correct super plug).

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Source: Carrier 59SC6A Installation, Start-up, Operating and Service Instructions, Fig. 60 Service Label Information, p.54. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.