Carrier Furnace 43 Error Code: Meaning, Causes & Fix

Low-heat pressure switch open while the high-heat pressure switch is closed — the stage switches disagree. That is the mainstream Carrier chart's wording (58CVA service label: 'LOW-HEAT PRESSURE SWITCH OPEN WHILE HIGH-HEAT PRESSURE SWITCH IS CLOSED'). On the older 58MVP variable-speed generation the same code is titled PRESSURE SWITCH CALIBRATION FAULT — see the honesty note in the what and FAQ: both titles are Carrier's own, on different generations.

A two-stage Carrier furnace proves its vent with two pressure switches set at different drafts: the low-heat switch makes at the gentle draft of low fire, and the high-heat switch needs the stronger draft of high fire before it will make. Physics therefore imposes an order — as the inducer ramps up, the low switch must close first, and any moment at which the high switch is closed, the low switch should certainly be closed too. Code 43 is the control catching that order broken: the high-heat switch has made while the low-heat switch is still open. The elegant consequence, and the reason this code repays understanding, is that the closed high-heat switch has already proven the vent and the inducer substantially innocent — a vent blocked badly enough to matter could never have made the needier switch. So 43 accuses the low-heat switch itself, its pressure tubing, or its wiring, which is exactly Carrier's check list: a low-heat pressure switch stuck open, disconnected or obstructed pressure tubing, and mis-wired pressure switches, with low inlet gas pressure added on models fitted with a low gas pressure switch (LGPS). This site's verified chart adds the condensing-furnace branches of the same story: a plugged condensate drain, or water standing in a sagging vent run, disturbing the pressure the low switch samples. The modulating generation names the same event with three switches (58MVC guide: 'LOW-HEAT PRESSURE SWITCH OPEN WHILE MEDIUM-HEAT PRESSURE SWITCH CLOSED') and documents the control's response: it shuts the unit down, inducer off and gas off, waits 2 minutes, and restarts the heating cycle. The older 58MVP manual's 'calibration fault' title describes the same relationship measured differently: during the high-heat purge the control reads the inducer RPM at which each switch makes, and faults 43 when the ratio falls outside 0.55 to 0.90 — the switches' make points are out of their calibrated relationship — or when the high switch simply makes before the low. Different generations, different instruments, one accusation: the low switch is not reporting the draft that the high switch proves is there.

What causes the 43 code on a Carrier furnace?

Low-heat pressure switch stuck open

Carrier's lead suspect and the logic of the code itself: the switch's contacts have failed to make at a draft the high-heat switch proves is more than sufficient. Switches die this way after years of cycling — the diaphragm stiffens or the contacts degrade until the switch simply never reports.

Disconnected or obstructed pressure tubing

The small hose carrying inducer/collector pressure to the low switch is off its port, cracked, kinked, or blocked with condensate debris. The switch then samples nothing — it is honestly reporting the pressure in a dead tube while the vent drafts perfectly. Carrier lists this check first for a reason.

Mis-wired pressure switches

The stage switches' leads crossed or landed on the wrong terminals, typically after a switch or control replacement — the control then reads the high switch's state where it expects the low's, and the 'disagreement' is in the wiring, not the vent. Carrier names mis-wiring explicitly on its 43 check list.

Plugged condensate drain or water in a sagging vent (condensing models)

From this site's verified Carrier chart: condensate that cannot drain, or water pooled in a low spot of the vent piping, disturbs the pressure at the sampling point and can hold the low switch out while the high switch — sampling through the stronger high-fire draft — still makes. This is the branch that makes 43 a winter, cold-snap code on condensing furnaces.

Low inlet gas pressure (LGPS-fitted models)

Carrier's parenthetical cause: on furnaces fitted with a low gas pressure switch, low inlet gas pressure enters the 43 check list. It is the minority branch — present only where an LGPS is fitted — but it is Carrier's, so it belongs on the list a technician works through.

Checks to run before you replace anything

1. SAFETY: never jumper or bypass a pressure switch to test heating, on this code above all — 43's whole logic depends on honest switch reports, and a bypassed switch destroys the evidence while defeating the vent proof. Switch testing is measured work with a manometer, and it is licensed-technician work.
2. Read the digit order twice through the sight glass: 43 is four short flashes, then three long. Its mirror 34 (ignition proving failure) is a different fault family entirely, and the two are confused constantly in owner reports.
3. Clear the outdoor vent and intake terminations anyway. The code's logic says the vent is substantially proven — but a partial restriction is cheap to rule out, and terminations are the one vent check that costs nothing and risks nothing.
4. Check the condensate trap and drain line on condensing models: a trap or drain you cannot pour water through freely is a documented 43 branch. Clear and flush it before anyone prices a switch.
5. Power off, then inspect the low-heat switch's tubing end to end: seated on both ports, no cracks, no sag holding water, no blockage at the collector-box tap. Reseat or replace perished tubing — this is the highest-value visual check on the code.
6. Note the pattern for the technician: does 43 strike on low-heat calls, during the ramp to high heat, or at purge before ignition? The 58MVC guide places the event at prepurge (the medium switch makes, the low does not, the control shuts down and waits 2 minutes) — when in the sequence your furnace faults is real diagnostic information.

How to fix carrier furnace error code 43?

1. Clear the condensate trap and drain and clear the vent/intake terminations; correct any sagging vent section that is holding water so condensate drains as designed.
2. Refit or replace the low switch's pressure tubing where it is disconnected, cracked or obstructed, and clear the pressure tap it samples from.
3. Correct pressure switch wiring to the furnace's diagram where the stage leads are crossed — then prove it through a full two-stage heating cycle.
4. Replace the low-heat pressure switch when measured draft at its tap exceeds its rating and it still will not make: the manometer reading, not the code, is what condemns the switch.
5. On 58MVP-generation furnaces, the technician's version of the same proof is Carrier's calibration test: the inducer RPM at each switch's make point, ratioed — a switch making far outside its calibrated relationship is replaced, not adjusted. Pressure switches are factory-set; there is no field calibration screw to turn, on any generation.

The part that usually fails: The low-heat pressure switch — and first, its tubing

The tubing is checked before the switch is priced because it produces the identical report for a fraction of the cost. The switch itself is replaced, never adjusted: its make point is factory-set, which is the entire substance behind the older generation's 'calibration fault' title.

How to reset carrier furnace error code 43?

43 is a self-recovering fault, not a lockout. The documented control behaviour (58MVC and 58MVP guides) is: the control shuts the unit down — on the 58MVP wording the inducer runs on briefly (about 15 seconds) as it shuts down, gas stays off — waits 2 minutes, stops flashing the code, and restarts the heating cycle on its own. No owner reset is needed or useful during that wait. A power cycle after the tubing, drain or wiring fix simply restarts the sequence sooner. The dividing line is repetition: a 43 that returns cycle after cycle has a standing cause (a dead switch does not revive between cycles), and at that point the reset conversation is over and the measurement conversation — draft at the tap versus the switch's rating — begins.

Carrier fault codes are stored by the control board and shown as two-digit codes via the status LED (short flash = first digit, long flash = second digit, with a pause between). Most codes clear automatically once the fault condition is gone and a new call for heat succeeds; a hard lockout (such as code 13 or 14) needs the fault fixed, then power cycled at the furnace switch or breaker for a few seconds, and in some lockouts the board waits up to an hour or requires several successful cycles before normal operation resumes. Clearing a lockout without fixing its cause just restarts the countdown to the next lockout.

Why does carrier furnace error code 43 keep coming back?

43 returning after a tubing and drain pass means the accusation has narrowed to the switch or the draft itself: a switch whose make point has drifted with age (the calibration story, arriving one winter at a time), or an inducer weakening so gradually that the low switch — the first to feel marginal draft at its tap — drops out while the high switch still catches the stronger purge. A technician separates those two with a manometer and, on the variable-speed generation, the inducer RPM readings Carrier's own calibration test uses.

How to stop 43 coming back

• Low-heat pressure switch stuck open: Carrier's lead suspect and the logic of the code itself: the switch's contacts have failed to make at a draft the high-heat switch proves is more than sufficient. Switches die this way after years of cycling — the diaphragm stiffens or the contacts degrade until the switch simply never reports.
• Disconnected or obstructed pressure tubing: The small hose carrying inducer/collector pressure to the low switch is off its port, cracked, kinked, or blocked with condensate debris. The switch then samples nothing — it is honestly reporting the pressure in a dead tube while the vent drafts perfectly. Carrier lists this check first for a reason.
• Mis-wired pressure switches: The stage switches' leads crossed or landed on the wrong terminals, typically after a switch or control replacement — the control then reads the high switch's state where it expects the low's, and the 'disagreement' is in the wiring, not the vent. Carrier names mis-wiring explicitly on its 43 check list.

Retrieving stored fault codes on Carrier furnaces

Carrier boards include a component test mode (a test pin/shorting jumper on the board, per the installation manual) that runs the inducer, blower and ignition sequence on demand, and the board keeps the last faults in memory readable as LED codes. The full fault chart is printed on the blower-door label of every unit — photograph it before relying on any online chart, because code assignments differ slightly between board generations.

Other Carrier furnace error codes

See the full list on the Carrier Furnace error codes hub.

Codes 43 gets confused with

33 — Limit circuit fault — a limit, flame rollout, draft safeguard or blocked-vent switch (as fitted) in the limit circuit is open right now, but the control hasn't escalated to lockout (13) yet. 33 is 13's earlier chapter, and Carrier's service instructions define it by the circuit, not just by overheating: a limit OR flame rollout switch is open.

21 — Gas heating lockout — Carrier's service label words it: 'STATUS CODE 21 — GAS HEATING LOCKOUT. Control will NOT auto reset. Check for: mis-wired gas valve; defective control (valve relay).' The control has shut down gas heating and, uniquely among Carrier's common lockouts, it will not clear itself with time. Code 21 is the Carrier control declaring that it cannot trust its own gas valve output.

13 — Limit circuit lockout — the control locked the furnace out because a limit or flame rollout switch in the limit circuit stayed open longer than 3 minutes, or limit trips came in succession (10 successive trips during high-heat; the variable-speed generation also locks out after 3 successive trips during low-heat). The control auto-resets after 3 hours. Carrier code 13 is a lockout, not a first offence, and Carrier defines its triggers exactly: a limit or flame rollout switch (on some models also a draft safeguard or blocked vent switch in the same circuit) open longer than 3 minutes, or successive limit trips — 10 during high-heat, and on the variable-speed generation 3 during low-heat.

32 — Low heat pressure switch did not close / opened (on two-stage Carrier furnaces: the low-fire pressure switch proof failed). Two-stage Carrier furnaces prove draft twice — once per firing rate, with separate pressure switches.

Frequently asked questions

Related guides

Facts on this page were checked against manufacturer documentation: mans.io · mans.io · www.shareddocs.com · mans.io · www.manualshelf.com · manualsdump.com