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Carrier · Furnace

43 Low-heat pressure switch open while high-heat switch closed

High severity 59TP6A

What the control is telling you

The control sees the high-heat pressure switch closed while the low-heat switch is open, which should not happen.

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

Likely causes

  • Low-heat pressure switch stuck open
  • Disconnected or obstructed pressure tube
  • Miswired pressure switches
  • Low inlet gas pressure (if LGPS used)
  • Plugged condensate drain
  • Water in vent pipe (sagging pipe)

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. Check the pressure switch wiring against the wiring diagram (Fig. 67).
  3. Inspect low-heat switch tubing for disconnection, blockage or water.
  4. Clear the condensate drain and check the vent for sagging sections holding water.
  5. Check inlet gas pressure if a low gas pressure switch is used.
  6. Replace the low-heat pressure switch if it stays open with correct pressure.

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Source: Carrier 59TP6A Installation, Start-up, Operating and Service Instructions, Fig. 66 Troubleshooting Guide (cont.), p.89. 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.