Furnace part
Pressure switch
The pressure switch proves the inducer is moving combustion air before the control allows gas to flow. Two-stage furnaces have two switches, one for each firing stage. A pressure-switch code usually points at venting, condensate or the inducer rather than at the switch itself.
The inducer creates a small negative pressure in the collector box; a hose carries that pressure to the switch, and when it reaches the set-point printed on the switch (fractions of an inch of water column), the contact closes and the control proceeds to ignition.
Because the switch only senses pressure, most of its fault codes trace to the things that change pressure: blocked or frozen vents, a flooded condensate trap, cracked hoses, or an inducer that cannot reach speed. The switch is tested with a manometer and replaced only if it fails that test.
Safety
Never bypass or jumper a pressure switch; it is the device that stops gas flow when the furnace cannot vent. Never adjust it.
Signs this part is involved
- Pressure-switch codes at start-up or mid-cycle
- Codes that appear only in cold weather or after heavy rain
- Furnace that stops when the condensate path backs up
Related fault codes
- Carrier 59TP6C status code 31: high-heat pressure switch (HPS) open or slow to close
- Carrier 59TP6C status code 32.2: open main pressure switch (LPS)
- Carrier 59TP6C status code 23: pressure switch stuck closed
- Carrier 59SC6A status code 31: secondary pressure switch open or lockout
Codes are documented per controller; check the model number on your rating plate before comparing. Not sure which control you have?
Related
- ComponentFurnace pressure switch
- SymptomFurnace not heating
- SymptomWater around a condensing furnace
- ServiceGas Furnace RepairFurnace repair in Greater Vancouver, diagnosed before parts are replaced
- ServiceFurnace DiagnosticsFind the actual fault before replacing parts: diagnosis by testing, not guessing
Questions people ask
My furnace shows a pressure-switch code in a cold snap. Is the switch bad?
Probably not: ice at the vent termination or a frozen condensate drain are the classic cold-weather causes. A manometer test against the switch's printed set-point settles it.
What do LPS and HPS mean?
On two-stage furnaces, LPS is the low-heat (main) pressure switch and HPS the high-heat (secondary) switch. Each has its own code on the control, documented on this site's code pages.
Not sure whether the pressure switch is your problem?
BC Wide diagnoses by testing, not by part swapping. Tell us the model number from the rating plate and what the furnace is doing.