Fix Carrier Furnace Code 31

A Complete, Step-by-Step Venting Solution That Actually Works

If your Carrier high-efficiency gas furnace is showing Error Code 31, this page is built to help you find the real cause and fix it correctly — not just replace parts and hope.

Code 31 is not a random failure.
It’s a venting and airflow condition problem.

What Does Furnace Code 31 Mean?

Code 31 = Pressure switch did not close, or opened during operation

In plain terms:

The inducer motor started

But the furnace could not confirm safe negative pressure

The system shut down to protect against unsafe combustion

This is a safety shutdown, not a control board error.

A Critical Clue Most Homeowners Miss

If your furnace:

Runs with the cabinet door open

But shows Code 31 when the door is closed

👉 This almost always means:

The furnace itself is fine.
The venting system is not.

Opening the door adds unintended airflow.
Closing it restores the real venting condition — and the pressure drops below spec.

The Real Causes of Code 31 (Ranked by Probability)
1️⃣ Incorrect PVC Vent Pipe Diameter (MOST COMMON)

This is the #1 root cause behind repeated Code 31 errors.

What goes wrong:

2” pipe used where 3” is required

Reducers installed mid-run

Intake and exhaust pipe sizes don’t match

Why it matters:

Smaller pipe = higher resistance

Higher resistance = lower negative pressure

Pressure switch never sees a safe condition

❗ If the pipe is undersized, no pressure switch replacement will fix Code 31.

2️⃣ Condensate Water in the Vent or Inducer

High-efficiency furnaces create condensate. Problems happen when:

Vent pipe slope is incorrect

Condensate trap is partially blocked

Water backs up into the inducer housing

Even a small amount of water can:

Reduce airflow

Distort pressure readings

Trigger Code 31 intermittently

3️⃣ Restricted Vent Termination (Outside the House)

Common restrictions include:

Snow, ice, leaves, debris

Vent caps with too small airflow openings

Flat or straight-out terminations facing wind

The pipe may look “open”, but the effective airflow area is reduced.

4️⃣ Pressure Switch Tubing Blockage

Water in the hose

Debris in the pressure port

Collapsed or brittle tubing

This often gets misdiagnosed as a “bad pressure switch”.

5️⃣ Inducer Motor Failure (Lowest Probability)

Only after vent size, slope, condensate, and termination are verified should the inducer motor be suspected.

Step-by-Step: How to Find the Problem
Step 1 – Verify Vent Pipe Diameter

Check the furnace installation manual

Measure the smallest diameter in the entire run

Count 90° elbows

Look for reducers

👉 If you’re unsure whether 2” is enough, it usually isn’t.

Step 2 – Check Condensate Drainage

Clean the condensate trap

Confirm free water flow

Listen for water inside the inducer

Step 3 – Inspect Outdoor Intake & Exhaust

Clear all obstructions

Ensure intake and exhaust are not too close

Avoid restrictive vent caps or fine mesh screens

Step 4 – Inspect Pressure Switch Tubing

Remove and confirm it’s dry

Clean the port fitting

Replace brittle tubing if needed

Why Vent Termination Design Matters (A Lot)

Many Code 31 problems come from stacked restrictions:

Slightly undersized pipe

Plus a restrictive vent cap

Plus cold weather condensate

That combination pushes the system past its pressure limit.

How LGQF-LL Solves the Problem (Without Creating New Ones)

LGQF-LL vent solutions are designed specifically to protect airflow, not restrict it.

Key design advantages:

✔ Large effective airflow opening

✔ 90-degree outlet reduces wind pressure and backflow

✔ Stainless steel screen prevents debris without choking airflow

✔ No reduction in pipe diameter

✔ Installs directly onto standard PVC pipe

Instead of masking the problem, it restores proper vent conditions — which is exactly what the pressure switch needs.

How to Test After the Fix

After correcting vent size, drainage, and termination:

Restore power

Call for heat

Observe:

Inducer starts smoothly

Pressure switch closes quickly

Ignition proceeds without delay

Run multiple heat cycles

Confirm no Code 31 returns

A stable system will:

Start consistently

Run with the cabinet door closed

Stay fault-free in cold weather

Final Advice from the Field

Code 31 is not a parts-replacement problem.
It’s a system-conditions problem.

Fix the vent diameter.
Fix the condensate path.
Fix the termination airflow.

Do those correctly — and Code 31 usually disappears for good.