How the fault is located
The cycle stops somewhere specific, and that is the diagnosis
A gas furnace is a state machine. The thermostat closes a circuit, the control board energises the inducer motor, the inducer establishes draft which closes the pressure switch, the board then energises the igniter, the gas valve opens, a flame sensor confirms the burner is alight, and after a delay the main blower starts. Every one of those steps has to complete before the next begins, and a fault stops the sequence at whichever step it belongs to.
That structure is what makes furnace diagnosis tractable. If the inducer never runs, the fault lies in the thermostat circuit, the board, the transformer, the door switch or the inducer itself, and nothing downstream is worth examining. If the inducer runs but ignition never starts, the pressure switch has not closed, which points at draft, a blocked pressure tap, water sitting in the condensate trap on a condensing appliance, or a genuinely failed switch. Working outward from the stop point is faster and considerably cheaper than the alternative.
The most common intermittent fault sits at flame proving. Once the burner lights, a flame sensor carries a tiny current through the flame to ground, in the order of microamps, and the board uses that current as proof of combustion. Oxide builds on the sensor over seasons and the current falls. At first the furnace lights normally. Later it lights, runs for a few seconds and shuts down, retries, and eventually locks out. The reading is measurable long before the symptom appears, which is exactly why it belongs on a seasonal service.
High limit trips deserve their own warning, because the limit switch is almost never the problem. That switch opens when the heat exchanger gets hotter than it should, and the usual reason is that too little air is passing over it. A loaded filter, a failing blower capacitor, a blower wheel clogged with debris, closed dampers or collapsed ductwork all produce the same trip. Replacing the limit switch removes the protection while leaving the overheating in place, which is how heat exchangers crack. We measure temperature rise across the exchanger and compare it against the range stamped on the appliance before touching the control.
| Where the cycle stops | What has not completed | Candidates worth testing |
|---|---|---|
| Nothing happens at all | The call never reached the board | Thermostat, wiring, transformer, door switch, blown low voltage fuse |
| Inducer runs, then stops | Pressure switch never closed | Blocked flue or intake, split pressure tubing, flooded condensate trap, failed switch |
| Igniter glows, no gas | Gas valve did not open or ignition timed out | Gas supply, valve coil, board output, igniter position |
| Lights then drops out in seconds | Flame proving failed | Flame sensor microamp reading, sensor position, grounding path |
| Runs then shuts down mid cycle | High limit opened | Filter, blower capacitor, blower wheel, duct restriction, temperature rise |
| Burner fine, no warm air | Blower never started | Blower motor, capacitor, board blower relay, fan delay setting |
| Short rapid cycling | Something is interrupting repeatedly | Oversized appliance, thermostat siting, flame signal marginal, rollout switch |
Nothing goes back into service without a combustion check. Flue carbon monoxide, oxygen, stack temperature and the temperature rise across the exchanger are recorded and compared against specification, and the heat exchanger is inspected where the symptom history gives any reason to. A furnace that runs is not the same as a furnace that is safe, and the difference is measurable.
Diagnostic order
Six steps that resolve most gas furnace faults
Followed in this order because each one eliminates everything downstream of it.
Read the board code
The diagnostic light is already reporting which stage failed. Reading it first costs nothing and removes most candidates.
Confirm the call
Twenty four volts present at the board, transformer healthy, low voltage fuse intact, door switch making.
Prove draft
Inducer operation, pressure switch closure and the state of the tubing, taps and condensate trap that feed it.
Watch ignition
Igniter resistance and glow, valve energisation, and the timing of the whole attempt against the appliance specification.
Measure flame signal
Microamps through the sensor, which distinguishes a dirty sensor from a grounding fault or a board input problem.
Check airflow and rise
Temperature rise across the exchanger against the stamped range, which is what most limit trips are really about.
On every call
What a furnace repair visit covers
Carried out regardless of the reported symptom, because several routinely change the diagnosis.
Fault code read and loggedIncluding the history where the board stores it, which often shows a pattern the homeowner never noticed.
Full combustion analysisCarbon monoxide, oxygen, stack temperature and efficiency, recorded before and after any work.
Temperature rise measuredAgainst the range stamped on the appliance, which is the honest test of airflow adequacy.
Safety chain verifiedLimit, rollout and pressure switches tested individually rather than assumed functional.
Burner and sensor conditionBurner ports, igniter position and flame sensor cleanliness, all of which drift slowly.
Written readingsEvery figure recorded, with anything trending toward a future failure flagged for you to schedule.
Customer testimonies
Feedback on gas furnace repairs
Previous company replaced the limit switch twice. This one measured the temperature rise and found the blower wheel packed solid with dust.
It had been cutting out for a month. The flame sensor was reading well under spec and the fix took ten minutes.
They read the stored fault history off the board and showed me it had been happening since before Christmas.
Furnace questions
Lockouts, limit trips, sensors and what is worth repairing
The furnace lights then shuts off after a few seconds, over and over. What is that?
That pattern is flame proving failing. The burner lights correctly, but the board does not receive enough current through the flame sensor to accept that combustion is happening, so it shuts the gas valve as a safety response, waits, and tries again. After a set number of attempts it locks out completely.
The usual cause is oxide accumulation on the sensor rod, and the fix is inexpensive. Less often it is a poor ground path, a cracked ceramic insulator, or a sensor that has drifted out of the flame envelope. All four are distinguished by measuring the actual microamp reading rather than by cleaning the sensor and hoping.
Should I just replace the limit switch if it keeps tripping?
No, and this is the single most damaging shortcut in furnace repair. The limit switch is a safety device that opens when the heat exchanger exceeds a safe temperature. It tripping is a symptom, not a fault, and the underlying cause is almost always insufficient airflow across the exchanger.
Replace the switch and the exchanger keeps overheating with nothing to interrupt it, which is a route to a cracked heat exchanger and combustion products entering the living space. The correct response is to measure temperature rise against the range on the data plate and find the airflow restriction, which is usually a filter, a blower capacitor, a fouled blower wheel or a duct problem.
How do I know if the heat exchanger is cracked?
It is not something to diagnose by eye alone, and it is not something to guess at either way. The indicators that justify a thorough inspection are flame disturbance when the blower starts, soot deposits, a persistent smell during operation, repeated unexplained limit trips, or a carbon monoxide detector that has activated.
Inspection involves a combustion analysis with the blower cycling, visual examination of accessible sections, and where the geometry allows, a camera. If a crack is confirmed the appliance is shut down and tagged rather than patched. That is not an upsell position, it is the only defensible one.
Is it worth repairing a furnace that is fifteen years old?
It depends entirely on which part failed. Igniters, flame sensors, pressure switches, capacitors, inducer motors and control boards are wear items, and replacing one on an otherwise sound appliance is usually good value even late in its life.
The calculation changes for a heat exchanger, and it changes for a unit that has needed three separate repairs in two seasons. In those cases you get the repair figure, the replacement figure and the reasoning together, and nobody here is compensated for steering you toward the larger one.
Related services
Where a furnace call commonly leads
Furnace faults frequently turn out to live in the distribution or the controls rather than in the appliance.
Boiler Repair
Boiler Replacement
Heat Pump Repair
Heat Pump Installation
Tell us how far into the cycle it gets
That one detail, plus whatever the diagnostic light is flashing, narrows a furnace fault to a handful of candidates before anybody arrives anywhere in Affton, MO.
The figure you approve before work begins is the figure on the invoice.