The check happens in the quiet second before anything spins. The control confirms the pressure switch is open at rest — proof the switch is honest. If it reads closed, the board cannot trust any later reading from it, so it refuses the cycle and posts 23. Carrier's manual gives two causes: obstructed pressure tubing, or the pressure switch stuck closed.
Because the fault is present at rest, the dramatic homeowner checks (snow on the vent, running condensate) do not apply — nothing has run yet. This is a switch-and-tubing diagnosis with the cabinet open, which on this site means a technician.
Code 23: Pressure switch did not open
Call a technician
Before each cycle the board confirms this switch is open. Manual check-for: obstructed pressure tubing, or the switch stuck closed.
From the table: Two-digit status codes — short flashes, then long flashes — source: Carrier 58YAV Service section - status code table (read the document, fetched 2026-10-30). Covers: Carrier 58-series and their Bryant/Payne twins. First digit = number of short flashes; second digit = number of long flashes. Code 33 is 3 short, pause, 3 long.
Why the board bothers
A pressure switch stuck closed would let the furnace fire with a blocked flue and never know — the proof-of-draft would be permanently "proven." Code 23 is the control refusing to fly with a gauge it cannot trust. Annoying, and exactly right.
Call a technician — the safe list:
- Run the basics once: thermostat calling for heat, filter clean, power switch on, blower door seated.
- If the code returns, that is the end of the homeowner list. This fault lives inside the cabinet, and the part it names needs testing with a meter.
The two rules that outrank everything on this page.
1. Smell gas? Stop reading. Leave the house and call your gas utility or 911 from outside. No code is worth diagnosing through a gas smell.
2. Never bypass a safety switch — no jumping a pressure switch, no taping a limit, no propping a rollout. Those switches are the only thing between a furnace fault and a fire or carbon-monoxide problem. This library explains codes; it does not teach repairs inside the cabinet.
One honesty note before the tables: codes change between control boards, sometimes inside the same brand and the same decade. Every table below names the board and the manual it was transcribed from. The chart glued inside your own blower door is the final word for your furnace — if it disagrees with this page, believe the door.