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Carrier · Furnace

23.1 Main pressure switch stuck closed

Medium severity 59SC6A

What the control is telling you

The main (collector box) pressure switch (LPS) is closed when it should be open. The control checks that both LPS and HPS are open before it starts the inducer on a heat call.

Applies to: Carrier 59SC6A

Likely causes

  • Welded/stuck pressure switch contacts
  • Jumpered or miswired pressure switch
  • Pressure tube issue causing a false signal

Test and repair steps

  1. With no inducer running, check continuity across the LPS: it must be open (it is a normally-open switch).
  2. Check for a jumper or miswiring at the switch terminals.
  3. If an LGPS accessory is not used, confirm its factory connection is intact and not miswired.
  4. Inspect the pressure tube for kinks or water.
  5. Replace the switch if it reads closed with the inducer off.
  6. Run a heat cycle and confirm the sequence starts.

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Source: Carrier 59SC6A Installation, Start-up, Operating and Service Instructions, Fig. 60 Service Label p.54; Fig. 61 Wiring p.56; Sequence of Operation p.68. 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.