Component
Furnace pressure switch
The pressure switch is a small diaphragm switch that proves the inducer is moving combustion air before the control allows gas to flow. Two-stage furnaces have two: a low-heat switch and a high-heat switch. A pressure-switch code usually points at venting, condensate or the inducer rather than at the switch itself.
Short answer
A pressure-switch fault means the control did not see the switch close, or saw it open, when it expected draft. Most causes are upstream of the switch: a blocked or frozen vent or intake, a flooded condensate trap, a cracked hose, or an inducer that is not reaching speed. The switch is tested with a manometer against its printed set-point and replaced only if it fails that test.
How it works
When the thermostat calls, the inducer motor starts and pulls combustion air through the burners and heat exchanger, creating a small negative pressure in the collector box. A hose carries that pressure to the switch; when it reaches the set-point printed on the switch (fractions of an inch of water column), the diaphragm closes a contact and the control proceeds to ignition.
On two-stage furnaces the inducer runs at two speeds and there are two switches. The low-heat switch must close for any heating; the high-heat switch must close for high fire. On Carrier's 59TP6C, code 32 covers the main (LPS) switch and code 31 the secondary (HPS) switch, with a specific minor code when the furnace falls back to low heat because the HPS was slow to close.
What causes pressure-switch codes
Vent and intake restrictions: snow, ice, leaves, nests, sagging pipe holding water, or a vent that exceeds the allowed equivalent length after a renovation.
Condensate: a trap or drain that is blocked lets water back up into the collector box and the pressure-switch port. Water in the hose is a classic winter no-heat call.
Inducer: a motor with worn bearings or a cracked wheel cannot pull enough draft, especially on high speed.
Hoses and ports: cracked, kinked or disconnected hoses and a collector-box port blocked with debris.
The switch: diaphragms do fail, but only after the above are ruled out with a manometer reading.
Documented codes that point here
- Carrier 59SC6A status code 23: pressure switch stuck closed
- Carrier 59SC6A status code 31: secondary pressure switch open or lockout
- Carrier 59SC6A status code 32.1: open main pressure switch (LPS)
- Carrier 59SC6A status code 33.1: a switch has opened in the main limit circuit
- Carrier 59SC6A status code 34: ignition fault or flame loss
- Carrier 59SC6A status code 13: limit lockout or ignition lockout
- Carrier 59SC6A status code 14: flame-loss lockouts during a heat request
- Carrier 59TP6C status code 31: high-heat pressure switch (HPS) open or slow to close
- Carrier 59TP6C status code 13: limit lockout or ignition lockout
- Carrier 59TP6C status code 14: flame-loss lockouts during a heat request
- Carrier 59TP6C status code 15.1: lockout - no blower rpms detected
- Carrier 59TP6C status code 23: pressure switch stuck closed
- Carrier 59TP6C status code 32.2: open main pressure switch (LPS)
- Carrier 59TP6C status code 33: limit circuit, high-heat or ignition lockout
- Carrier 59TP6C status code 34: flame lost after ignition
- Carrier 59TP6C status code 43.1: HPS closed before LPS
- Carrier 59TP7A status code 31: high-heat pressure switch (HPS) open or slow to close
- Carrier 59TP7A status code 13: limit lockout or ignition lockout
- Carrier 59TP7A status code 14: flame-loss lockouts during a heat request
- Carrier 59TP7A status code 15.1: lockout - no blower rpms detected
- Carrier 59TP7A status code 23: pressure switch stuck closed
- Carrier 59TP7A status code 32.2: open main pressure switch (LPS)
- Carrier 59TP7A status code 33: limit circuit, high-heat or ignition lockout
- Carrier 59TP7A status code 34: flame lost after ignition
- Carrier 59TP7A status code 43.1: HPS closed before LPS
- Carrier 59SP6B status code 23: pressure switch stuck closed
- Carrier 59SP6B status code 31: secondary pressure switch open or lockout
- Carrier 59SP6B status code 32.1: open main pressure switch (LPS)
- Carrier 59SP6B status code 33.1: a switch has opened in the main limit circuit
- Carrier 59SP6B status code 34: ignition fault or flame loss
- Carrier 59SP6B status code 13: limit lockout or ignition lockout
- Carrier 59SP6B status code 14: flame-loss lockouts during a heat request
- Carrier 59SP6B status code 15.1: lockout - no blower rpms detected
- Carrier 59TN7A status code 31: high-heat pressure switch (HPS) open or slow to close
- Carrier 59TN7A status code 13: limit lockout or ignition lockout
- Carrier 59TN7A status code 14: flame-loss lockouts during a heat request
- Carrier 59TN7A status code 23: pressure switch stuck closed
- Carrier 59TN7A status code 32.2: open main pressure switch (LPS)
- Carrier 59TN7A status code 43.1: HPS closed before LPS
- Carrier 59TN7A status code 33.1: a switch has opened in the main limit circuit
- Carrier 59TN7A status code 34: ignition fault or flame lost after ignition
Related
Questions people ask
Can I clean or bypass the pressure switch myself?
Do not bypass it; it is the device that stops gas flow when the furnace cannot vent. Cleaning the port and hose is a technician's job on a gas appliance, and the underlying cause still needs to be found.
Why does it only happen in cold weather?
Ice at the vent termination, condensate freezing in a low spot of the vent, and frozen condensate drains all appear in cold snaps and all show up as pressure-switch codes.
Need this looked at?
Call BC Wide with the brand, model number and anything on the display. Or
Request service online with this page attached.