What It Means
Contradictory pressure switch readings between low-heat and high-heat switches.
Carrier Furnace
Carrier Furnace Error Code 43
Troubleshooting guide for the Carrier Furnace system fault 43.
Possible Causes
- Low manifold gas supply pressure.
- Clogged condensate drain or water in vent piping.
- Crossed or incorrect pressure switch wiring.
- Blocked pressure switch sensing tubing.
Not sure which one it is — or if it's none of these? Try ACLogics today.
Disclaimer: This information is for general reference. Error codes can vary by model and production year. Always refer to your machine's specific manual and the information printed on the unit itself.
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Full Diagnosis
Low-Heat Pressure Switch Open While High-Heat Pressure Switch Is Closed. The control detects a contradictory switch state — the low-heat pressure switch reads open at the same moment the high-heat switch reads closed — and holds the heating call until the two readings agree. This differs from codes 31 and 32, which each flag a single high-heat or low-heat switch that failed to close or reopened during its own stage call; code 43 instead compares both switches against one another rather than judging either in isolation. Root causes include low inlet gas pressure at the manifold, a plugged condensate drain, improper pressure-switch wiring, water pooled in the vent piping, or obstructed pressure tubing. Unlike the timed lockouts on codes 13, 14, and 45, this fault carries no fixed lockout duration — the furnace resumes staging as soon as both switches report consistent states.
All Recorded Causes
Check low inlet gas pressure at the manifold. Verify condensate drain is clear and draining properly. Inspect pressure switch wiring connections for looseness or a crossed connection between the two switches. Check for water accumulation in vent piping. Clear any obstructed tubing. Confirm the inducer is producing adequate vacuum for the high-heat stage. If both switches test correctly on their own, compare wiring against the diagram — a wiring swap between the two switches can mimic this dual-switch mismatch, which is a different failure mode than the single-switch faults in codes 31/32.
Verify Before Replacing Parts
- Confirm the exact Carrier model family and furnaces context before acting on a generic code description.
- Walk the entire heating sequence in order instead of jumping straight to the reported code, watching for check low inlet gas pressure at the manifold.
- Verify each safety device and prove where the sequence stops under live operation, including whether verify condensate drain is clear and draining properly.
- After the correction, run the system long enough to confirm the furnaces fault does not return under normal load.
Still unsure about this fault?
ACLogics walks you through Carrier Furnace diagnostics step-by-step — so you confirm the cause before swapping parts.
