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Carrier · Furnace

24 Secondary (24 V) fuse open

Medium severity 59TP6A58SB0A/58SB1A

What the control is telling you

The 3-amp automotive-type fuse in the 24-V circuit on the control has blown.

Applies to: 59TP6A two-stage variable-speed ECM multipoise condensing gas furnace, all sizes (040-10 to 120-22)

Likely causes

  • Short in 24-V wiring including thermostat leads
  • Short in outdoor unit low-voltage wiring

Test and repair steps

  1. Warning: Shut off 115-V power and gas, and lock/tag out before working inside the furnace; there may be more than one power source.
  2. Disconnect the thermostat leads at the control to isolate the short.
  3. Check thermostat cable and outdoor unit wiring for damage or shorts to ground.
  4. Replace the fuse with a 3-amp fuse ONLY.
  5. Reconnect leads one at a time and confirm the fuse holds through heat and cool cycles.

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Source: Carrier 59TP6A Installation, Start-up, Operating and Service Instructions, Fig. 66 Troubleshooting Guide (cont.), p.89; p.76. Summarized in our own words; check the original manual for your model.

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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.