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Carrier gas furnace code 43: Lower pressure switch open while a higher one is closed
Gas furnace, Canada and United States
The low-heat pressure switch (the low or medium switch on modulating furnaces) is open while a higher pressure switch is closed.
On the status light, this code is 4 short flashes, then 3 long flashes.
This needs a licensed gas technician. There is nothing safe to check yourself. Leave the covers on and book a professional.
This involves the flue or exhaust. A blocked or leaking flue can let carbon monoxide into the home.
Only some models show this code. Check the list below.
What causes it
-
The low-heat (or low or medium) pressure switch is stuck open.
-
Disconnected or blocked pressure tubing, or mis-wired pressure switches.
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On condensing furnaces, a plugged condensate drain or water lying in a sagging vent pipe.
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Low inlet gas pressure, on furnaces fitted with a low gas pressure switch.
How to read this code on your unit
The amber status light (red on some older boards) on the furnace control shows through a small window in the blower access door. A steady light means the control has power and no fault to show. A code is two digits. Count the short flashes for the first digit, then the long flashes for the second, so 3 short flashes followed by 3 long flashes is code 33, and 1 short then 1 long is code 11. Write the code down before you open the door or switch the furnace off.
Booking a licensed gas technician
Book a licensed gas technicianlicensed gas technician
- Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
- Note the code, how often it appears, and the model and serial numbers from the rating label.
- Book a licensed gas technician and give them those details.
Models that show this code
114 models in 11 ranges
| Range | Models |
|---|---|
| 58CTA/CTX | 58CTA045-08, 58CTA045-12, 58CTA070-08, 58CTA070-12, 58CTA070-16, 58CTA090-14, 58CTA090-16, 58CTA090-20, 58CTA110-12, 58CTA110-16, 58CTA110-22, 58CTA135-16, 58CTA135-22, 58CTA155-20, 58CTX045-08, 58CTX045-12, 58CTX070-08, 58CTX070-12, 58CTX070-16, 58CTX090-14, 58CTX090-16, 58CTX090-20, 58CTX110-12, 58CTX110-16, 58CTX110-22, 58CTX135-16, 58CTX135-22, 58CTX155-20 |
| 58CVA/CVX | 58CVA070-12, 58CVA090-16, 58CVA110-20, 58CVA135-22, 58CVA155-22, 58CVX070-12, 58CVX090-16, 58CVX110-20, 58CVX135-22, 58CVX155-22 |
| 58TN0A/58TN1A | 58TN0A045C17-12, 58TN0A070C14-12, 58TN0A070C17-16, 58TN0A070C21-20, 58TN0A090C17-16, 58TN0A090C21-20, 58TN0A110C21-20, 58TN0A135C24-22, 58TN1A045C17-12, 58TN1A070C14-12, 58TN1A070C17-16, 58TN1A070C21-20, 58TN1A090C17-16, 58TN1A090C21-20, 58TN1A110C21-20, 58TN1A135C24-22 |
| 58TP0A/58TP1A | 58TP0A045V14-12, 58TP0A070V14-12, 58TP0A070V17-16, 58TP0A090V17-16, 58TP0A090V21-20, 58TP0A110V21-22, 58TP0A135V24-22, 58TP1A045V14-12, 58TP1A070V14-12, 58TP1A070V17-16, 58TP1A090V17-16, 58TP1A090V21-20, 58TP1A110V21-22, 58TP1A135V24-22 |
| 59MN7A | 59MN7A060V17-14, 59MN7A080V17-14, 59MN7A060V21-20, 59MN7A080V21-20, 59MN7A100V21-22, 59MN7A120V24-22 |
| 59MN7B | 59MN7B060C17-14, 59MN7B060C21-20, 59MN7B080C17-14, 59MN7B080C21-20, 59MN7B100C21-22, 59MN7B120C24-22 |
| 59TN6A | 59TN6A060V17-14, 59TN6A080V17-14, 59TN6A080V21-20, 59TN6A100V21-22, 59TN6A120V24-22 |
| 59TN6B | 59TN6B060C17-14, 59TN6B080C17-14, 59TN6B080C21-20, 59TN6B100C21-22, 59TN6B120C24-22 |
| 59TP5A | 59TP5A040E14-10, 59TP5A060E14-12, 59TP5A040E17-12, 59TP5A060E17-14, 59TP5A080E17-16, 59TP5A080E21-20, 59TP5A100E21-20, 59TP5A120E24-22 |
| 59TP6A | 59TP6A040V14-10, 59TP6A040V14-12, 59TP6A060V17-12, 59TP6A060V17-14, 59TP6A080V17-16, 59TP6A080V21-20, 59TP6A100V21-20, 59TP6A120V24-22 |
| 59TP6B | 59TP6B040V14-10, 59TP6B040V17-12, 59TP6B060V14-12, 59TP6B060V17-14, 59TP6B080V17-16, 59TP6B080V21-20, 59TP6B100V21-20, 59TP6B120V24-22 |
Where this comes from
- 58CTA/CTX two-stage induced-combustion gas furnace, installation, start-up, operating and service instructions (58CT-12SI) , page 45
- 59TP6A two-stage variable-speed condensing gas furnace, installation, start-up, operating and service instructions (59TP6A-10SI) , page 73, 90
- 59MN7A modulating condensing gas furnace, installation, start-up, operating and service instructions (59MN7A-12SI) , page 72, 90
Checked against the manufacturer's manual on 11 October 2026.
Other Carrier gas furnace codes
| Code | Meaning | What you can do |
|---|---|---|
| 23 | Flue or venting fault (depends on model) | Call a pro |
| 31 | Flue or venting fault (depends on model) | Safe checks first, then a pro |
| 32 | Low or low-heat pressure switch did not close, or opened again | Safe checks first, then a pro |
| 42 | Inducer motor fault | Safe checks first, then a pro |
| 14.4 | Lockout after repeated pressure switch openings | Safe checks first, then a pro |
| 23.1 | Flue or venting fault (depends on model) | Call a pro |
| 23.2 | Secondary pressure switch stuck closed | Call a pro |
| 23.3 | Pressure transducer senses over-pressure | Call a pro |
| 23.4 | Low pressure switch stuck closed | Call a pro |
| 23.5 | High pressure switch stuck closed | Call a pro |
| 23.6 | Medium pressure switch stuck closed | Call a pro |
| 43.1 | High switch closed before the low switch | Call a pro |
See all flue or venting fault codes on gas furnaces across brands.