Furnace Heater Sensors: The Complete DFW Homeowner Guide

Does your HVAC system feel lonely this holiday season? Get ready to learn all about your furnace sensors...

What They Do, Why They Fail, and When to Replace Them

If your furnace keeps shutting off, blowing cold air, or refusing to ignite, there’s a very good chance a furnace heater sensor is behind it. Modern furnaces rely on multiple safety sensors to protect your home, your equipment, and your family — but the most common one to fail is the flame sensor, and honestly, the poor thing deserves a medal.

Let’s break down every major furnace sensor, how they work, what causes them to fail, and which signs tell you it’s time for repair before you lose heat on a cold North Texas morning.

This guide is built from real-world failures we see every winter across Allen, Plano, Frisco, McKinney, Carrollton, Lewisville, and Far North Dallas.

What Is a Furnace Heater Sensor?

A furnace heater sensor is any safety device inside your furnace that monitors heat, flame, airflow, or temperature. When something unsafe happens, these sensors shut the system down instantly to prevent:

• House fires

• Carbon monoxide poisoning

• Overheating

• Heat exchanger cracks

• Gas buildup

A furnace can have 3–6 sensors depending on brand, model, and age.

The Most Important Furnace Sensors

Flame Sensor (Most Common Failure)

This is the star of the show — and the sensor that fails the most.

What it does

Confirms there is an actual flame when the gas valve opens.

Why it fails

Because this dude sits in the flame the entire time your heater runs.

Heat + carbon buildup = corrosion + failure.

Symptoms of a bad flame sensor

• Furnace lights, then shuts off in 1–10 seconds

• System tries to restart over and over

• Cold air blowing

• Burner lights inconsistently

• Furnace lockout

Fix

Cleaning sometimes works. Replacement is often better, especially on older units.

High Limit Sensor (High Limit Switch)

This one sits near the heat exchanger.

Its job

Shuts the furnace down if temperatures get dangerously high.

Why it trips

• Dirty filter

• Closed vents

• Undersized ductwork

• Poor airflow

• Blower motor problems

• Heat exchanger blockage

What homeowners feel

• Furnace works for 2–5 minutes then shuts off

• Burning smell

• Short cycling

• Overheating messages on the thermostat

Rollout Switches (Rollout Safety Sensors)

Located near each side of the burners or near the flue pipe.

What they protect from

Flame rollout — when flame leaves the burner area due to blockage, cracked heat exchanger, or improper combustion. This is extremely dangerous.

Why rollout switches trip

• Blocked heat exchanger

• Soot buildup

• Improper gas pressure

• Blocked vent or flue pipe

• Cracked heat exchanger

• Flame instability

Rollout trips are never a “just reset it” situation.

Pressure Switch

This monitors proper airflow through the flue.

Why it matters

If the furnace cannot push gases safely up and out of the house, the system locks out.

Signs of pressure switch issues

• Furnace won’t ignite

• Clicking noise but no flame

• Error codes for drafts or airflow

Inducer Motor Sensors / Draft Safety Sensors

These verify that flue gases are moving properly.

If the furnace can’t vent carbon monoxide safely, this sensor stops the entire process.

Never Remove a Furnace Safety Sensor or Replace It with a Non-Factory Part

This is where homeowners get into trouble.

Removing or bypassing furnace safety sensors can cause

• A house fire

• Carbon monoxide poisoning

• Gas explosions

• Cracked heat exchanger

• Total system failure

• Voided warranties

• Failed home inspections

Why factory specs matter

Sensors must match exact temperature tolerances. A slightly “hotter” or “cooler” aftermarket switch creates major risk.

If a safety trips repeatedly, the sensor is doing its job. Find the root cause — don’t bypass it.

Why Furnace Heater Sensors Fail More Often in North Texas

Homes in Allen, Plano, and Frisco experience:

• Heavy dust

• Attic-installed systems exposed to extreme heat

• Long run times

• Undersized ductwork

• High-limit trips caused by airflow issues

So sensors work overtime — and fail faster.

DIY Checks Before Calling a Tech

You can safely check these at home:

• Replace the filter

• Ensure all vents are open (1-2 closed are okay)

• Inspect flame sensor for visible soot

• Look for flashing error codes

• Listen for inducer motor operation

• Make sure the flue pipe isn’t blocked with debris

If the system keeps shutting down, it’s time to call a pro.

How HVAC Techs Diagnose Sensor Problems

The Gurus have NATE Certified technicians, but who cares! The important thing is they are trustworthy and honest.

We check:

• Microamp signal on flame sensor

• Temperature readings at heat exchanger

• Burner alignment and flame stability

• Pressure switch vacuum readings

• Rollout switch continuity

• Inducer motor draw and venting

• High limit temperature curves

• Real-time airflow and static pressure

Most furnace sensors fail due to a root cause, not the part itself.

When to Replace vs When to Repair

Replace if

• Flame sensor is corroded and cleaning fails

• Rollout switch tripped due to real unsafe conditions

• High limit switch is heat-damaged

• Sensor is out of factory spec

• The furnace is 10-15+ years old & Repairs over $500

  • The Rule of 5000: multiple the age times the repair cost and if it exceeds $5K replace; unless your tech is pushy. Then call me and will install it for you : )

Repair if

• Dirty flame sensor

• Blocked ductwork

• Plugged flue

• Dirty blower wheel

• Clogged filter

• Restricted return air

Serving DFW Homeowners Trusted AC Repair and Installation

This content is optimized for: Allen • Plano • Frisco • McKinney • Carrollton • Dallas • The Colony • Lewisville • Prosper • Murphy • Wylie

Need Furnace Sensor Repair?

Heat and Air Gurus can diagnose the real cause, protect your home, and document everything with video.

Call or text 469-797-1269 Book online at GurusAir.com Call the Cow, Say WOW.

FAQ

What is a furnace heater sensor?

A furnace heater sensor is any safety device that monitors flame, temperature, or airflow to prevent dangerous conditions.

Why does my furnace keep shutting off?

You may have a bad flame sensor, a tripped high limit switch, a rollout switch activation, or a pressure switch issue.

What is the most common furnace sensor to fail?

The flame sensor. It sits inside the flame, building up soot and corrosion over time.

Is it safe to bypass furnace sensors?

Absolutely not. Bypassing safety sensors risks fire, carbon monoxide poisoning, and system damage.

What causes the high limit sensor to trip?

Dirty filters, restricted airflow, overheating, blower problems, or undersized ductwork.

What does a rollout switch do?

It stops the furnace if flames leave the burner area — a sign of dangerous combustion.

How can I tell if my flame sensor is bad?

If your furnace lights briefly then shuts down within seconds, your flame sensor might be dirty or failing. To clean you need a 1/4" hex head and a nailfile or dollar bill. Turn off power. Remove flame sensor. Be careful not to remove the ignitor as those can be more delicate. Gently scrub with low grit nailfile or dollar bill. Replace and see if burners stay lit. Remember Heat and Air Gurus when your need a new Furnace or AC unit.