Carrier 59SC2B Error Code 23: Pressure Switch Did Not Open

What Does Code 23 Mean?

Status code 23 (two short flashes, three long flashes) on the Carrier 59SC2B indicates the pressure switch failed to open when the control expected it to. The pressure switch is a draft-safety device that should rest in the open position and only close once the inducer establishes adequate draft. Before that draft exists — and after the inducer stops — the board expects to see the switch open. Code 23 means it was found closed at the wrong time.

There are two typical explanations. The pressure tubing that carries draft signal to the switch may be obstructed (by condensate, insects, or debris) so that pressure stays applied to the diaphragm and holds the switch closed. Or the switch contacts themselves may be mechanically stuck closed. Either way the board cannot verify the safety interlock is functioning, so it refuses to proceed.

It is important not to confuse code 23 with code 31. Code 23 is the switch failing to OPEN when it should be open; code 31 is the opposite — the switch failing to CLOSE, or reopening, during an actual draft problem. Because verifying and replacing a pressure switch and clearing its tubing require a manometer and proper testing, code 23 is a professional diagnosis rather than a DIY fix.

What You'll Notice

• The furnace will not begin its ignition sequence and produces no heat
• The draft/inducer behavior looks abnormal because the board already sees the switch closed at rest
• The fault may first appear after service that disturbed the pressure switch or its tubing
• Code recall shows a two-short, three-long pattern

Common Causes

How This Is Diagnosed

A technician first checks the pressure switch's resting state, which should read open with the inducer off; finding it closed points either to the switch or its tubing. The pressure tubing is then inspected for obstruction or trapped condensate, and the switch is tested with a manometer to confirm it opens and closes at the correct pressure thresholds. A switch that stays closed after the tubing is verified clear is replaced.

When to Call a Professional

This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:

• The furnace will not start its ignition sequence and is showing code 23
• The code appeared after any work involving the pressure switch or its tubing
• The switch or tubing needs to be tested or cleared, which requires a manometer and proper procedure

✓ Verified against manufacturer service manual — March 2026