Coleman Furnace 3 Flash Code: Pressure Switch Stuck Open

Quick Answer

A 3 flash code means your furnace is not detecting enough vacuum pressure to safely vent exhaust gases. Usually, this is caused by a blocked vent pipe, a clogged pressure switch port, or a failing inducer motor rather than a bad switch. The furnace will stay in a lockout mode and refuse to ignite for safety.

Look, when this code fires your furnace is locked out and it won't restart on its own until you deal with the root cause. I've seen people ignore it for days thinking it'll sort itself out. It won't. And if there's a real venting issue, you're potentially dealing with CO backing up into the house. Most of the time it's something dumb like a blocked port or a cracked hose, not a $40 switch.

ColemanFurnaceSeverity: highDifficulty: intermediate

Time to Fix

30–60 min

Difficulty

intermediate

Parts Cost

—

Tools Needed

Phillips #2 screwdriver, Flat-head screwdriver

What Does the 3 Flash Code Code Mean?

Here's the deal. The furnace tried to start, the inducer motor spun up, and then the control board waited for the pressure switch to confirm good airflow before letting any gas in. It never got that confirmation. The whole thing shut down on purpose. Good news is this usually isn't expensive. A pressure switch runs $15 to $40. But honestly, most of the time you don't even need a new one because the switch isn't actually what's broken.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Blocked vent or intake pipes45%

Clogged pressure switch port or hose30%

Failing inducer motor or capacitor15%

Faulty pressure switch10%

Symptoms You May Notice

You hear the inducer motor kick on for about 30 to 60 seconds, then the whole furnace shuts completely down before you ever smell gas or hear the igniter click.
The diagnostic LED on the control board blinks exactly 3 times, pauses for a couple seconds, then repeats over and over until you deal with it.
Your house is getting colder even though the thermostat is definitely calling for heat and you can clearly hear the furnace trying to start.
Inducer starts up fine but then sounds like it's surging or losing speed right before shutdown, which usually means water in the vacuum line or a partially blocked vent termination.

Can you reset a Coleman furnace to clear the 3 Flash Code code?

Find the power switch on the side of the furnace cabinet, it looks like a regular light switch mounted on a gray metal box near the unit. Flip it off and wait a full 30 seconds. Then flip it back on with your thermostat still calling for heat. Give it 2 to 3 minutes to run through the complete startup sequence. If the 3-flash code comes right back, the underlying cause isn't fixed yet and you need to work through the diagnostic steps above.

Tools Required for Diagnosis

Phillips #2 screwdriverFlat-head screwdriverDigital multimeter (AC voltage and continuity/ohms modes)Flashlight or headlampStraightened paperclip or thin stiff wireWet/dry shop vac (for clearing condensate lines)Needle-nose pliers

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

Frequently Asked Questions

Can I just bypass the pressure switch to get heat?

You can jump it for about 30 seconds as a diagnostic test to confirm the rest of the system fires up, but that's it. Don't leave it jumped. That switch is there to confirm exhaust gases are actually leaving your house before the gas valve opens. If there's a real venting blockage and you bypass the switch, you're basically telling the furnace to pump combustion gases into your living space. Carbon monoxide poisoning is not how you want to spend a cold Tuesday night. Find the actual cause and fix it.

Why is there water in my pressure switch hose?

This happens on high-efficiency condensing furnaces when the secondary heat exchanger or condensate drain lines are clogged or kinked. The furnace produces a ton of water vapor as a byproduct of combustion, and that condensate has to drain somewhere. When it can't drain fast enough, it backs up into the inducer housing and gets sucked right into the vacuum port. The fix is usually clearing the condensate trap, that little U-shaped plastic fitting hanging below the heat exchanger, or checking that the drain hose isn't kinked, cracked, or frozen solid.

Does a 3 flash code always mean I need a new pressure switch?

Rarely, honestly. I'd say maybe one out of every six or seven calls I go on for this exact code actually ends with a new switch. The switch is almost always doing exactly what it's supposed to do, which is reporting a real airflow problem somewhere else in the system. Replacing the switch without first checking the vent pipes, the inducer motor speed, and that little vacuum port on the inducer housing is just guessing with your wallet. Work through the simple stuff first.

How do I reset the Coleman 3 flash code?

Flip the power switch on the furnace off for 30 seconds, then back on. That clears the lockout and lets it attempt the startup sequence again. But here's the thing, if you haven't fixed whatever caused the code, it'll be back within a minute or two when the furnace tries and fails again. The reset is really only useful after you've addressed the problem, or as a quick first step to watch how early in the startup sequence it's failing.

How much does fixing this usually cost?

If it's just a blocked vent cap or a clogged orifice on the inducer, you're spending zero dollars and maybe 20 minutes of your afternoon. Pressure switch replacement runs $15 to $40 for the part. A new inducer motor is where it gets real money, usually $150 to $350 for the part alone. If you're calling a tech out, expect $100 to $200 for the diagnostic and a basic repair, more if they have to come back with parts. Most of the time this code is a cheap fix if you catch it before something else fails trying to compensate.

Models Known to Experience 3 Flash Code Errors

This repair applies to most Coleman furnaces with this error code. Common model numbers include:

TG9S040A12UP11, TG9S060B12UP11, TG9S080C16UP11, TG9S100D20UP11, TC1S040A12UP11, TC1S060B12UP11, P9MPN0601412A

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 17, 2026