On the covered Carrier gas furnace, 43 means the low-heat pressure switch did not make during a call for high heat while the high-heat switch did. The control keeps flashing 43; if the low-heat switch still has not closed after 5 minutes it shuts the unit down and waits 15 minutes before restarting. First, look for anything blocking the vent terminations outside, such as snow, ice, leaves or a nest.
Differential
Likely causes of “Carrier gas furnace code 43”, and how to tell them apart
- 1
A low-heat pressure switch that did not make
What separates it Code 43 flashes during a call for high heat
How to confirm Carrier's flowchart points at the switch, its tubing or its wiring
- 2
A blocked vent outside
What separates it The inducer runs but the switch will not close
How to confirm Check the terminations
- 3
A low gas pressure switch fault
What separates it The control flashes 43 and then shuts down
How to confirm Technician testing
- 4
A wire or harness that is open
What separates it Intermittent behavior
How to confirm Technician repair
- 5
A control fault
What separates it The parts test well
How to confirm Technician replacement
Before you touch a setting
Record this first — changing settings destroys the evidence
- That the code reads 43
- Whether it appears on a high heat call
- Whether the vent terminations are clear
- The model number
Try in this order
0 of 8 completedWhat to do first
ReversibleLook for anything blocking the vent terminations outside, such as snow, ice, leaves or a nest.
Correct only the documented fault domain
ReversibleThis combination points at the low-heat switch, its tubing and wiring, or the low gas-pressure switch. Carrier's flowchart ends at repairing an open wire or harness, replacing the low-heat pressure switch or assembly, replacing the low gas-pressure switch, or replacing the furnace control.
Look at the vent terminations
ReversibleOutside.
Clear any snow or debris
ReversibleFrom the openings.
Wait out the shutdown
ReversibleCarrier says 15 minutes.
Let it restart
ReversibleAnd watch one cycle.
Note whether it repeats
ReversibleAnd when.
Call a technician
ReversibleIf it returns.
Model-specific
What is true of this model and not of its siblings
- Two switches
- Carrier's two-stage furnaces use a low-heat and a high-heat pressure switch. In high heat, both should close. Code 43 means the high-heat switch closed and the low-heat did not, which is odd and points at the low-heat switch itself.
- What Carrier's flowchart ends at
- Repairing an open wire or harness, replacing the low-heat pressure switch or assembly, replacing the low gas-pressure switch or replacing the control.
- The timing
- The control keeps flashing 43; if the switch does not close in five minutes, it shuts down and waits 15 minutes.
- Outside checks
- A blocked termination can show up as a pressure fault, so look before you call.
- Why not bypass
- Jumping the switch hides a vent problem.
- Records
- Write down the flash time and the weather.
- Why this combination is unusual
- In high heat, the inducer runs faster and both pressure switches normally close. If the high-heat switch closes while the low-heat does not, the physics suggests a problem with the low-heat switch or its connection, not the vent, because the vent must be drawing for the high-heat switch to close. That is why Carrier's flowchart goes to the switch, tubing and wiring.
- The 15 minute wait
- Carrier's control keeps flashing 43 and, after five minutes without the low-heat switch closing, shuts the unit down and waits 15 minutes before trying again. If the furnace seems to be off for a quarter of an hour at a time, this is why.
- What to tell a technician
- Say that the code is 43, whether it appears only on high heat calls, and whether the furnace is a two-stage model. Note the weather, since moisture in the tube can cause the switch to stick in cold air.
Stop boundary
When this stops being a self-service repair
- !Never bypass, jumper or tape over a safety switch
- !Stop for a carbon monoxide alarm, soot, scorch marks or flame outside the burner compartment
- !Call a qualified technician for anything behind the furnace's panels
- !If you smell gas, leave the building and call the gas utility from outside; do not use switches or phones inside
Evidence ledger
Sources behind this answer
Exact questions
Common questions
What does 43 mean on a Carrier gas furnace?+
The low-heat pressure switch did not make during a call for high heat while the high-heat switch did. The control keeps flashing 43; if the low-heat switch still has not closed after 5 minutes it shuts the unit down and waits 15 minutes before restarting.
What happens after five minutes?+
The control shuts the unit down for 15 minutes.
Can I fix it?+
Check outside; the rest is technician work.
Never jumper+
A pressure switch proves the vent is drawing.