Carrier Furnace Error 43 Solved: Detailed DIY Repair

• Difficulty: Intermediate – Requires basic electrical testing and mechanical disassembly.
• Estimated Time: 45 to 90 minutes.
• Tools Needed: Phillips head screwdriver, Digital Multimeter, small wire brush or a paperclip, and needle-nose pliers.
• Estimated Cost: $0 (if it’s just a clog) to $150 (if the switch or control board needs replacement).

Carrier Furnace Error 43 indicates a Pressure Switch Calibration Fault. This specific code triggers when the furnace control board detects that the pressure switch is closed at a time when it should be open, usually during the initial start-up sequence or after a cycle ends. Essentially, the “brain” of your furnace thinks the safety sensors are lying to it.

If you’re seeing this, you’re likely dealing with a house that’s getting colder by the minute, a furnace that hums but refuses to ignite, or a blower fan that runs incessantly without producing a lick of heat. Take a breath—I’ve spent thirty years in crawl spaces dealing with this exact issue. It’s frustrating, but it’s a logic error that we can usually trace back to a physical blockage or a worn-out component. We’ll get it sorted.

The Complete Solution

Step 1: Power Down and Safety First. Before you touch a single wire, flip the service switch on the side of the furnace or turn off the breaker. Dealing with 120V and a control board isn’t the time for shortcuts. Open the upper and lower cabinet panels by unscrewing the Phillips head screws or releasing the latches. Set them aside where they won’t get stepped on.

Step 2: Inspect the Pressure Switch Tubing. Locate the pressure switch—it’s a small, round plastic disc with one or two rubber hoses attached to it. Pull the hoses off (remember which goes where). Check them for cracks, brittle spots, or water. If there’s water inside, blow through them to clear it out. Safety Warning: Do not blow into the pressure switch itself with high pressure; you can rupture the delicate internal diaphragm.

Step 3: Clear the Inducer Port. This is the “old mechanic’s secret.” Take a small paperclip or a thin piece of wire and gently poke it into the metal or plastic nipple where the hose connects to the furnace’s large plastic inducer housing. Often, a tiny bit of scale or a “mud dauber” nest blocks this hole, trapping pressure and causing the calibration fault. Make sure that hole is clear and unobstructed.

Step 4: Test for Continuity. Grab your digital multimeter and set it to the “Ohms” or “Continuity” setting. With the wires removed from the pressure switch and the furnace OFF, touch your leads to the switch terminals. It should read “Open” (or infinite resistance). If it shows “Closed” (0 Ohms) while the furnace is off, the switch is defective and must be replaced. This is the “calibration” failure the board is complaining about.

Step 5: Check the Condensate Trap. If you see water backed up in the hoses, you need to find the plastic trap (usually a white or black box on the side). Flush it with warm water and a bit of bleach to clear out any slime. If water can’t drain, the air pressure in the furnace won’t be right, and Error 43 will keep coming back like a bad penny.

Step 6: Reassemble and Re-test. Reconnect the hoses snugly. Reattach the wires to the switch. Put the doors back on (most Carriers have a door switch that must be depressed for the unit to power up). Flip the power back on. The furnace should go through a “self-test,” the inducer should kick on, and within 30-60 seconds, you should see the glow of the igniter. If it fires up, you’ve nailed it.

Why is my Carrier showing Error 43?

In my experience, Error 43 isn’t usually a “broken” part right out of the gate; it’s often a “confused” part. Here are the deep-dive reasons why this happens:

1. Stuck Pressure Switch Contacts: Over years of thousands of cycles, the internal diaphragm or the electrical contacts inside the pressure switch can “weld” shut or become physically stuck. If the control board sees the switch is already “closed” (meaning it thinks there is airflow) before the inducer motor even starts spinning, it knows something is wrong and shuts down for safety. This is often caused by simple mechanical wear and tear.

2. Blocked Condensate Lines or “Sludge”: If you have a high-efficiency condensing furnace, moisture is a byproduct. If the plastic drain lines or the “collector box” get backed up with gunk or algae, water can back up into the pressure switch tubing. This moisture creates a “false” pressure reading or prevents the switch from resetting to the “open” position when the furnace turns off.

3. Obstructions in the Port (The “Spider” Factor): I can’t tell you how many times I’ve found a tiny insect or a bit of corrosion inside the “port”—the little nipple where the rubber hose connects to the inducer housing. Even a microscopic blockage here prevents the vacuum from releasing, keeping the switch in a “calibrated closed” state when the board expects it to be open.

4. Control Board Logic Failure: While less common, voltage spikes or age can degrade the relays on the furnace control board. If the board can no longer accurately “read” the resistance from the switch, it defaults to Error 43 as a fail-safe.

Symptoms

When a Carrier unit throws Error 43, it doesn’t just stop; it talks to you through a series of physical “protests.” First and foremost, you will see the amber or red LED on the control board flashing—look for 4 short flashes followed by 3 long ones. This is your definitive confirmation.

Beyond the light, you’ll notice the Inducer Motor (that small fan that clears out exhaust) might kick on, but the igniter never glows, and the burners never catch fire. The system will eventually go into a “watchguard” or lockout mode. You might also hear a faint clicking sound as the relays try to engage, followed by the main blower fan running at high speed to clear out any perceived “dangerous” gases, blowing cold air through your vents and making the house feel even chillier.

How to Prevent Error 43

Don’t just fix it once; keep it from happening again. Here is how you stay ahead of the curve:

Annual Port Cleaning: Every autumn, before the first cold snap, pull that rubber hose and poke the port with a paperclip. It takes ten seconds and prevents 80% of pressure switch calls. While you’re there, make sure no birds have built nests in your PVC exhaust pipes outside.

Maintain Your Drain Lines: If you have a high-efficiency unit, pour a cup of white vinegar through the condensate drain line once a year. This prevents the “sludge” buildup that causes water to back up into the pressure sensors.

Install a High-Quality Surge Protector: Since Error 43 involves the control board’s “calibration” logic, protecting that board from “dirty” power or lightning strikes is vital. A dedicated HVAC surge protector can save you a $500 board replacement down the road.

Frequently Asked Questions

Q: Can I just jump the wires to bypass the switch?
A: Absolutely not. I’ve seen houses burn down because of that. The pressure switch is a safety device that ensures exhaust gases (carbon monoxide) are being vented out of your home. If you bypass it and the vent is blocked, those gases stay in your living room. Never bypass a safety switch.

Q: Is Error 43 the same as Error 31?
A: Close, but no. Error 31 usually means the switch won’t close when it should. Error 43 means the switch won’t open or is out of its expected sequence. They both involve the same system, but 43 is more about the “resting state” or calibration of the switch.

Q: How long do these switches usually last?
A: In a perfect world, 10 to 15 years. However, if your furnace is in a damp basement or has poor drainage, the internal components can corrode in as little as 5 years. If you’ve replaced yours twice in three years, you have a moisture problem, not a part problem.