Lennox Furnace Code 33 usually means the pressure switch circuit is open or not proving airflow when the inducer motor should be running. The single most likely fix is clearing a blocked pressure-switch hose or venting restriction, then confirming the inducer and switch tubing are dry, tight, and undamaged.

⚡ Quick Fix

1. Shut off power to the furnace at the switch or breaker, or unplug it if your setup allows. Wait 15 to 30 minutes so the control board can fully reset, then restore power and call for heat at the thermostat. If Code 33 returns, move to the airflow and pressure-switch checks.
2. The most common real fix is to inspect the pressure switch hose and venting path. Remove any water, debris, or cracks in the rubber tubing, and make sure the intake and exhaust pipes are not blocked by leaves, snow, nests, or condensate buildup. A restricted vent or leaking hose prevents the pressure switch from closing.
3. If the simple checks do not solve it, test the inducer assembly and pressure switch circuit. Verify the inducer starts, inspect the wiring harness and terminals, and use a multimeter to confirm the pressure switch changes state when the inducer pulls draft. If the switch does not respond or the inducer is weak, deeper diagnosis is needed.

What This Error Means

On a Lennox Furnace, Error Code 33 typically points to a pressure switch problem during the ignition sequence. The control board expects the inducer motor to create negative pressure in the heat exchanger and venting system, which closes the pressure switch. If that signal does not reach the PCB, the furnace stops the heating cycle to prevent unsafe combustion or flue gas spillage.

Technically, this fault can be caused by an open pressure switch, a blocked intake or exhaust pipe, condensate trapped in the pressure tubing, a weak inducer motor, or damaged wiring between the switch and control board. In high-efficiency condensing furnaces, water in the collector box, drain trap, or vent can also interfere with pressure readings. The fault is often electrical and airflow-related at the same time, so both sides need to be checked.

Because the pressure switch is a safety device, do not bypass it to make the furnace run. A furnace that cannot prove proper draft may have a venting restriction, inducer failure, or control issue that can lead to repeated shutdowns or unsafe operation. The goal is to find out why the switch is not proving, not just to clear the code.

Common Causes

Blocked vent pipe or intake

Leaves, insect nests, snow, ice, or debris in the vent termination can reduce inducer draft enough to keep the pressure switch open. Even a partial restriction can trigger Code 33, especially during cold or wet weather.

Cracked, loose, or water-filled pressure switch hose

The small rubber or silicone hose between the inducer housing and pressure switch must hold vacuum properly. If it is split, disconnected, brittle, or filled with condensate, the pressure switch cannot sense draft correctly.

Weak or failing inducer motor

If the inducer wheel is dirty, the motor bearings are worn, or the capacitor is weak on designs that use one, draft may be too low to close the switch. You may hear humming, slow startup, or an abnormal rattling noise before the error appears.

Faulty pressure switch or wiring harness

A pressure switch can fail mechanically or electrically after years of cycling. Corroded terminals, a loose spade connector, damaged wiring harness, or poor continuity back to the control board can also create the same fault code.

Condensate drain or collector box issue

On condensing Lennox furnaces, a clogged drain trap or collector box can alter pressure in the combustion path. Water backing up into the inducer housing or tubing often causes intermittent pressure-switch faults that appear and disappear.

Step-by-Step Fix

Tools you may need: screwdriver, multimeter, flashlight

Safety warning: Disconnect power before opening any panels or touching internal components.

1. Reset the furnace and watch the startup sequence. Restore power and set the thermostat to call for heat, then observe whether the inducer motor starts before the code appears. If the inducer never starts, stop here and check power supply, door switch engagement, fuse on the control board, and board output before continuing.
2. Inspect the intake and exhaust venting from the furnace to the outside termination. Look for snow, leaves, bird nests, sagging pipe, ice, or signs of condensate pooling. Clear any restriction you find, and make sure vent joints are intact and not leaking air.
3. Remove and inspect the pressure switch hose. Check for cracks, brittleness, soft spots, kinks, and water inside the tubing, then clear it and reinstall it firmly on both ports. Also inspect the pressure tap on the inducer or collector box because that small port can clog with debris and prevent the switch from seeing proper vacuum.
4. Check the condensate system if your Lennox Furnace is a condensing model. Inspect the drain trap, drain lines, and collector box for blockage or standing water, and flush the trap if needed. If water is backing up into the inducer housing or pressure tubing, correct the drain issue before replacing any electrical parts.
5. Test the pressure switch circuit with a multimeter. With power off, inspect the wiring harness and terminals for corrosion, looseness, or heat damage; with power on and the inducer running, verify the switch changes continuity as draft is established. If the switch remains open despite clear venting and good draft, the switch may be defective.
6. Evaluate inducer performance. The inducer should start quickly, run smoothly, and pull steady draft without excessive noise or vibration. If it struggles to start, runs slowly, or has a damaged wheel or failing motor/capacitor, stop and call a professional because the assembly may need electrical testing and replacement.
7. Inspect the control board only after the airflow and switch checks are complete. Look for a blown fuse, burnt traces, or loose low-voltage connections that could interrupt the pressure switch signal to the PCB. If all field components test normally but Code 33 remains, a technician should verify board input/output and combustion draft with proper instruments.

Related Errors

• Error 31 — Pressure switch did not close, usually caused by inducer, venting, or pressure-switch circuit issues.
• Error 34 — Ignition proving or burner operation fault, often tied to gas flow, flame sensing, or sequence problems.
• Error 41 — Blower motor fault or limit on operation, commonly related to airflow or indoor fan control issues.
• Error 201 — Lost flame sense during operation, indicating intermittent flame signal or burner stability problems.
• Error 223 — Low pressure switch did not close, pointing to draft or venting failure on compatible Lennox models.

FAQ

Can I reset this error without a technician?

Yes, you can try a basic reset by shutting off power to the furnace for 15 to 30 minutes and restarting it. If the code comes back, the reset did not fix the underlying problem, and you should check the venting, pressure switch hose, condensate drain, and inducer operation.

Is Lennox Furnace Code 33 dangerous?

It is a safety-related fault, so you should take it seriously. Code 33 means the furnace is not proving proper draft through the pressure switch circuit, which can be caused by blocked venting or combustion-air problems. The furnace usually locks out to prevent unsafe burner operation.

Can I bypass the pressure switch to make the furnace run?

No. The pressure switch is a critical safety device that confirms the inducer is moving combustion gases correctly. Bypassing it can allow the furnace to run with a blocked vent, poor draft, or flue gas hazard, and it can also damage components or create a much more expensive repair.

How much does it cost to fix Code 33?

The cost depends on the cause. A simple vent cleaning, hose replacement, or condensate drain service is usually on the lower end, while inducer motor or control board work is more expensive. Labor rates vary widely, but diagnosis is often straightforward if the technician can reproduce the fault.

Why does the code appear only in cold or wet weather?

Cold or wet conditions can expose marginal venting and condensate problems. Snow, ice, heavy wind, or extra moisture can block vent terminations, fill pressure tubing with water, or slow drainage in condensing furnaces. That changes the pressure signal enough for the control board to post Code 33.

When should I stop troubleshooting and call a professional?

Call a professional if the inducer does not start, the pressure switch does not change state during testing, the furnace repeatedly locks out, or you find signs of burnt wiring, water in electrical compartments, or damaged venting. Combustion diagnostics and pressure readings should be verified with proper instruments.