Carrier Furnace Code 43: Pressure Switch Calibration Fault
Get this fixed by a local pro
Free, no-obligation quotes from licensed HVAC technicians in your area. Compare up to 3 estimates before anyone visits.
Carrier furnace code 43: what it means
Carrier Furnace Code 43 is a pressure switch calibration fault. On variable-speed condensing furnaces, the control board ramps the inducer motor to specific speeds and watches the low-pressure switch to confirm it opens and closes at the expected points. This “calibration” lets the board account for real-world venting resistance.
If the switch changes state at the wrong inducer speed — too early, too late, or not at all — the board cannot complete calibration and posts Code 43. The root cause is almost always venting, the pressure-switch tubing, or the switch itself, not the gas or ignition system.
Because the furnace typically will not fire until calibration succeeds, this is a pro-level fault: it protects against operating with restricted or unsafe venting. The board usually retries on the next heat call, but the code will keep returning until the physical cause is corrected.
Note: exact code numbering and wording differ between board generations. Confirm the meaning on the fault-code legend printed inside your blower compartment door before ordering parts.
Carrier furnace code 43: common causes, ranked by probability
- Blocked or restricted vent/intake termination — snow, ice, leaves, insect nests, or a bird screen clogged at the exterior PVC pipes changes the pressure the inducer sees.
- Pressure-switch tubing problems — cracked, kinked, disconnected, or condensate-filled hoses give the switch a false reading during the calibration sweep.
- Blocked or sagging condensate drainage — water backing up in the collector box or trap raises venting resistance and skews calibration.
- Failed or out-of-spec low-pressure switch — a switch that trips at the wrong point (or sticks) can’t be calibrated against.
- Inducer motor weak or out of range — a variable-speed inducer that can’t hit its target speeds prevents the board from finding a valid calibration.
- Vent pipe sizing/length or joint leaks — improper installation or a loose coupling changes system pressure enough to fail calibration.
- Control board fault — least common; the board’s calibration logic or sensing circuit is faulty.
How to fix Carrier furnace code 43: safe checks first
These are the only steps a homeowner should perform:
- Check the thermostat — confirm it’s set to Heat and the temperature is calling. Replace batteries if it’s a battery model.
- Replace a dirty air filter — a badly clogged filter can disturb airflow and pressures.
- Inspect exterior vent and intake pipes — from outside, make sure the PVC terminations aren’t blocked by snow, ice, leaves, or nests. Clear anything visible from the outside only.
- Check the condensate line — look for a clogged or overflowing drain or full trap; clear standing water at the accessible end if you can do so safely.
- Reset once — cycle the furnace switch or breaker off, wait 30 seconds, and turn it back on for one retry.
- Seat the blower door — make sure the access panel is fully closed; an ajar panel can disturb the door switch. Do not remove the panel — only confirm from the outside that it is latched.
If Code 43 returns after one reset, stop and call a technician. Do not open the cabinet or touch the pressure switch, tubing, or inducer.
How a technician will diagnose it
A qualified tech will typically:
- Read the stored fault history and confirm Code 43 against the board legend.
- Inspect vent and intake piping end-to-end for blockage, slope, length, and leaking joints.
- Check the pressure-switch tubing for cracks, kinks, disconnection, or trapped condensate.
- Measure switch operation with a manometer, verifying it opens/closes at the rated pressure. There is no single universal value — the rated set point is printed on the switch body and varies by model, so it has to be read off the part in your furnace.
- Verify the inducer motor reaches its commanded speeds and draws normal current.
- Inspect the condensate collector box and trap for restriction.
- Confirm proper draft, then command a calibration cycle and watch the switch response.
- Replace the pressure switch, tubing, inducer, or board only after isolating the fault — not by guesswork.
Sanity check: a good quote names the specific failed component after testing, not a blanket board swap.
Symptom, cause and what to do
| Symptom | Likely cause | DIY action | Technician job |
|---|---|---|---|
| Code 43, furnace won’t fire | Blocked vent/intake terminations | Clear visible snow/debris from outside pipes | Inspect full venting, remeasure draft |
| Code 43 after cold/windy weather | Ice or nest in exterior pipes | Clear only what’s visible outside | Inspect terminations, verify slope/length |
| Code 43 with water near furnace | Blocked condensate drain | Clear accessible drain end | Clean collector box and trap |
| Code 43, inducer runs but won’t validate | Cracked/kinked/full switch tubing | None — internal | Inspect and replace tubing |
| Code 43 persists after clearing vents | Failed pressure switch | None — internal | Manometer test, replace switch |
| Code 43 with weak/noisy inducer | Inducer out of speed range | None — internal | Test and replace inducer motor |
| Code 43 with all else good | Control board fault | None — internal | Verify sensing circuit, replace board |
Carrier furnace code 43 repair cost
Honest US ranges (parts + labor; regional variation applies):
- DIY safe checks (filter, clearing vent/condensate blockage): $0–$40
- Pressure-switch tubing replacement: $120–$250
- Low-pressure switch replacement: $150–$350
- Condensate trap/collector cleaning: $120–$300
- Vent repair or re-piping: $200–$700+ depending on scope
- Inducer motor replacement: $500–$1,200 installed — variable-speed/ECM inducer assemblies on condensing furnaces sit at the upper end of this band
- Control board replacement: $500–$1,200 — variable-speed/Infinity boards are among the pricier furnace parts
- Diagnostic/service call: $90–$180, often credited toward the repair
Simple blockages cost little; component replacement is where bills climb. Always get the failed part identified by test before authorizing an inducer assembly or a board.
Related codes
- Carrier Furnace Code 31: Causes, Fixes & Costs — on two-stage and variable-speed boards, the high-pressure (high-heat) switch did not close, or reopened during operation. Don’t confuse it with Code 32, which concerns the low-pressure switch.
- Carrier Furnace Code 32: Low-Pressure Switch Fault & Fixes — closely related low-pressure switch issue.
- Carrier Furnace Code 23: Pressure Switch Did Not Open — the opposite switch-state fault.
- Carrier Furnace Code 42: Inducer Motor Fault & Fixes — for inducer speed problems that can affect calibration.
- Carrier Furnace Code 33: Causes, Fixes & Costs — a limit circuit fault, not a pressure circuit fault.
Frequently asked questions
Does Carrier Code 43 reset by itself?
On most variable-speed boards the furnace retries calibration on the next call for heat, but a persistent fault holds until the underlying venting, tubing or switch problem is fixed. Repeated cycling without repair is a symptom, not a solution.
Can I fix Code 43 myself?
Only the safe basics — check for blocked vent terminations, a clogged condensate line and a dirty filter. Calibration involves the inducer, pressure switch and tubing inside the cabinet, which is technician work.
Is Code 43 dangerous to run with?
The furnace usually will not fire until it calibrates, which is a protective behavior. Do not bypass the pressure switch or force operation; a calibration fault can point to a venting blockage that must be corrected safely.