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Carrier gas furnace code 45.4: Microprocessor communication lost
Gas furnace, Canada and United States
The control microprocessor lost communication internally. To clear it, cycle the power; if it comes back, the control must be replaced.
This needs a licensed gas technician. There is nothing safe to check yourself. Leave the covers on and book a professional.
Only some models show this code. Check the list below.
What causes it
InteliSense display furnaces, InteliSense modulating furnaces
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The control microprocessor lost communication internally. A technician cycles the power, and replaces the control if the code repeats.
InteliSense ultra-low NOx furnaces
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The control microprocessor lost communication. A technician cycles the power, and replaces the control if the code repeats.
How to read this code on your unit
The three-digit display sits on the furnace control behind the blower door. Through the sight glass in the door you see only the status light, which flashes the first two digits of the code. 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 a 33.x code. Write down what you see before you open the door or switch the furnace off. The full code with its third digit shows on the control display only with the door removed and the door switch closed, which is a job for a technician. A technician can also look back at the last 7 faults in the FLt menu. Stored codes are erased by themselves after 72 hours of powered operation after the last fault, and the display shows non when no fault is stored.
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
83 models in 10 ranges
| Range | Models |
|---|---|
| 58SP0B/58SP1B | 58SP0B045V14-12, 58SP0B070V14-12, 58SP0B070V17-16, 58SP0B090V17-16, 58SP0B090V21-20, 58SP0B110V21-22, 58SP0B135V24-22, 58SP1B045V14-12, 58SP1B070V14-12, 58SP1B070V17-16, 58SP1B090V17-16, 58SP1B090V21-20, 58SP1B110V21-22, 58SP1B135V24-22 |
| 58TN0B/58TN1B | 58TN0B045C17-12, 58TN0B070C14-12, 58TN0B070C17-16, 58TN0B070C21-20, 58TN0B090C17-16, 58TN0B090C21-20, 58TN0B110C21-20, 58TN0B135C24-22, 58TN1B045C17-12, 58TN1B070C14-12, 58TN1B070C17-16, 58TN1B070C21-20, 58TN1B090C17-16, 58TN1B090C21-20, 58TN1B110C21-20, 58TN1B135C24-22 |
| 58TP0B/58TP1B | 58TP0B045V14-12, 58TP0B070V14-12, 58TP0B070V17-16, 58TP0B090V17-16, 58TP0B090V21-20, 58TP0B110V21-22, 58TP0B135V24-22, 58TP1B045V14-12, 58TP1B070V14-12, 58TP1B070V17-16, 58TP1B090V17-16, 58TP1B090V21-20, 58TP1B110V21-22, 58TP1B135V24-22 |
| 59TN6C | 59TN6C040C14-10, 59TN6C060C17-14, 59TN6C080C17-14, 59TN6C080C21-20, 59TN6C100C21-22, 59TN6C120C24-22 |
| 59TN7A | 59TN7A060C17-16, 59TN7A080C17-16, 59TN7A080C21-20, 59TN7A100C21-22, 59TN7A120C24-22 |
| 59TP6C | 59TP6C040V14-10, 59TP6C040V17-12, 59TP6C060V14-12, 59TP6C060V17-16, 59TP6C080V17-16, 59TP6C080V21-20, 59TP6C100V21-20, 59TP6C100V21-22, 59TP6C120V24-22 |
| 59TP7A | 59TP7A060V17-16, 59TP7A080V17-16, 59TP7A080V21-20, 59TP7A100V21-20, 59TP7A100V21-22, 59TP7A120V24-22 |
| 58CU0B | 58CU0B040C17-12, 58CU0B060C17-16, 58CU0B080C21-20, 58CU0B100C21-20 |
| 59CU5B | 59CU5B060C17-16, 59CU5B080C21-20, 59CU5B100C21-22 |
| 59MN7C | 59MN7C060C17-14, 59MN7C060C21-20, 59MN7C080C17-14, 59MN7C080C21-20, 59MN7C100C21-22, 59MN7C120C24-22 |
Where this comes from
- 58SP0B gas furnace, installation, start-up, operating and service instructions (58SP0B-01SI) , page 30
- 59TP6C gas furnace, installation, start-up, operating and service instructions (59TP6C-03SI) , page 56
- 59TN6C gas furnace, installation, start-up, operating and service instructions (59TN6C-04SI) , page 57, 76
- 58TP0B gas furnace, installation, start-up, operating and service instructions (58TP0B-01SI) , page 49
- 58CU0B gas furnace, installation, start-up, operating and service instructions (58CU0B-03SI) , page 28, 43
- 59CU5B gas furnace, installation, start-up, operating and service instructions (59CU5B-05SI) , page 49, 66
- 59MN7C gas furnace, installation, start-up, operating and service instructions (59MN7C-04SI) , page 53, 74
Checked against the manufacturer's manual on 11 October 2026.
Other Carrier gas furnace codes
| Code | Meaning | What you can do |
|---|---|---|
| 45 | Control circuitry lockout | Call a pro |
| 42.2 | Inducer speed lost while running | Call a pro |
| 42.3 | Inducer over its speed limit | Call a pro |
| 43.1 | High switch closed before the low switch | Call a pro |
| 45.1 | Internal control fault | Call a pro |
| 45.2 | Gas valve relay will not close | Call a pro |
| 45.3 | Control memory (EEPROM) fault | Call a pro |
| 46.1 | Momentary loss of power | Call a pro |
| 51.1 | Outdoor air sensor open | Call a pro |
| 51.2 | Outdoor air sensor shorted | Call a pro |
| 52.1 | Supply air sensor open | Call a pro |
| 52.2 | Supply air sensor shorted | Call a pro |
See all control board fault codes on gas furnaces across brands.