If you’re seeing this, your furnace is likely “short-cycling”—starting the inducer motor, failing to ignite the burners, and then shutting down. You’ll notice the house getting colder, a repetitive humming or clicking sound followed by silence, and the lack of that familiar “whoosh” of flames. Don’t panic; while it’s a lockout, it’s usually caused by a simple blockage or a worn-out component that we can diagnose and fix without calling in a whole crew.
| Metric | Details |
|---|---|
| Difficulty | Moderate – Requires basic electrical testing and mechanical disassembly. |
| Estimated Time | 45 – 90 Minutes |
| Tools Needed | Phillips/Nut Driver, Digital Multimeter, Small Wire or Paperclip, Soft Rubber Tubing. |
| Estimated Cost | $0 (Cleaning) — $150 (New Pressure Switch) |
Symptoms of a Pressure Switch Cycle Lockout
When your Lennox furnace hits an Error 6 code, it won’t just sit there quietly. You will notice the small LED on the control board blinking red six times in a specific sequence, followed by a brief pause. Physically, you’ll hear the inducer fan (the small motor) kick on, but the igniter never glows, and the gas valve never opens.
🛠️ Safety Precaution: High Voltage
For your safety and to avoid voiding the warranty, please check the official docs.
Another common symptom is “hunting,” where the furnace tries to start three or four times, stays on for only a few seconds, and then gives up, leaving you with cold air blowing through the vents. You might also hear a faint “clicking” sound, which is the pressure switch diaphragm struggling to make a solid electrical connection. If the unit is located in a basement, you might notice an unusual amount of moisture or “sloshing” sounds near the furnace, indicating a drainage issue related to the pressure sensor’s environment.
Detailed Diagnosis: Root Causes
In my thirty years of turning wrenches, I’ve learned that a “bad switch” is rarely the actual problem—it’s usually the messenger. Here is why that switch is failing to stay closed:
- Flue Pipe or Intake Obstruction: This is the most common culprit. High-efficiency furnaces use PVC pipes to breathe. If a bird builds a nest in the intake or if snow/ice blocks the exhaust, the inducer motor cannot create the vacuum pressure needed to “pull” the switch closed. Without proper airflow, the furnace won’t fire because it could lead to carbon monoxide backup.
- Clogged Condensate Collector Box: High-efficiency furnaces produce water (condensate). If the drain lines or the collector box are plugged with “gunk” or scale, water backs up into the inducer housing. This water prevents the pressure switch from sensing the correct air pressure, leading to a lockout.
- Perished or Blocked Pressure Tubing: The small rubber hose connecting the inducer to the switch can dry out and crack over time due to the heat. Even a pinhole leak will prevent the vacuum from holding the switch closed. Alternatively, moisture can get trapped inside the tube, creating a “vapor lock” that confuses the sensor.
- Mechanical Diaphragm Failure: Inside that plastic switch is a thin rubber diaphragm and a set of electrical contacts. Over thousands of cycles, the rubber can become stiff or the contacts can “pitting” or corrode. When this happens, even if the pressure is correct, the electrical circuit isn’t completed.
Comprehensive Repair Guide
Follow these steps in order. We want to start with the easiest, free fixes before we start buying parts.
- Safety First: Before you touch anything, shut off the electrical power to the furnace at the service switch or the breaker panel. Then, turn the gas valve to the “Off” position. You’re working around high-voltage components and moving fan blades.
- Inspect the Venting: Go outside and look at your PVC intake and exhaust pipes. Ensure no leaves, ice, or debris are blocking them. Back inside, check the internal venting for any signs of sagging or water pooling. A sagging pipe can trap water and block airflow just enough to trigger a Code 6.
- Clear the Pressure Port: Locate the small rubber tube leading from the pressure switch to the inducer motor. Pull the tube off the motor side. Take a thin piece of wire or a paperclip and gently poke it into the small hole (the port) on the motor housing. These ports often get clogged with a tiny bit of rust or scale. Clearing this often restores the vacuum signal instantly.
- Inspect the Tubing: Pull the rubber hose completely off. Inspect it for cracks, brittle ends, or water inside. If there is water, blow it out. If it’s cracked, you must replace it with heat-rated silicone tubing. A secure, airtight fit is non-negotiable here.
- Test for Continuity: This is where the multimeter comes in. Set it to the Ohms (Ω) setting. With the furnace off and the wires disconnected from the switch, the switch should show “Open” (No continuity). Now, while the inducer motor is running (after you turn the power back on), the switch should “Close” (Show 0 Ohms). If the motor is screaming but the switch stays “Open,” the switch is likely shot or the motor isn’t pulling enough vacuum.
- Check the Condensate Drain: Ensure the plastic trap and the drain lines are clear. If you see water standing in the inducer fan housing, you have a drainage blockage. Clear the lines using a shop-vac or by flushing them with warm water and a little white vinegar to kill any algae growth.
How to Prevent Error 6 Flashes Red
You don’t want to be doing this again next February. To keep the pressure switch happy, focus on these two preventive measures:
1. Annual Inducer Port Cleaning: Every autumn, before the first cold snap, pull that rubber hose and clear the port with a paperclip. It takes 30 seconds and prevents 90% of the “no-heat” calls I get. Dust and oxidation are inevitable; don’t let them build up.
2. Install Protective Vent Screens: If your furnace vents out the side of the house, install mesh bird-guards. Small rodents and birds love the warmth of the exhaust pipe and can crawl inside, causing a permanent blockage that is a nightmare to clear. Just make sure the mesh is wide enough not to allow ice to “frost over” the opening in extreme cold.
FAQ
Frequently Asked Questions
Can I “jump” the pressure switch to get heat tonight?
Absolutely not. The pressure switch is a safety device that ensures your furnace isn’t venting poison gas into your living room. Jumping the switch bypasses this safety and could lead to a fire or carbon monoxide poisoning. If it won’t close, there is a dangerous reason why.
Does a Code 6 always mean the furnace is broken?
Not necessarily. Often, it’s an external factor like a heavy snowstorm blocking the vent or a clogged drain line. It’s the furnace doing exactly what it was designed to do: shut down before a minor problem becomes a catastrophe.
How do I reset the lockout once I’ve fixed the problem?
Most Lennox boards will reset themselves after a successful power cycle. Turn the power switch off for 60 seconds and then back on. If the issue is fixed, the board will clear the Error 6 and attempt a fresh ignition sequence.