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HVAC Diagnostic Guide HIGH URGENCY

Furnace Keeps Shutting Off

Furnace lights and runs for several minutes, then shuts itself off — different from short cycling.

Quick Check Before You Call a Pro

  1. Note how long furnace runs before shutting off — 5 min vs 20 min suggests different causes
  2. Replace air filter — restricted airflow trips limit switch
  3. Check the LED status light on the control board for blink codes
  4. Look for any unusual smells (gas, burning) during operation

01 · Most Likely Causes (Ranked)

35%

Dirty Flame Sensor (Most Common)

What's happening

Flame sensor is oxidized and can't detect the flame after a few minutes. Gas valve closes as a safety, burners shut off. Blower runs to cool the heat exchanger, then everything restarts. Can repeat 3 times before control board locks out.

How to check

Furnace runs 5-10 minutes, burners shut off (you can see through inspection window), blower keeps running, then ignition attempts again.

How to fix

Turn off power AND gas. Locate flame sensor (thin metal rod in burner flame area). Remove single screw, slide out. Clean rod GENTLY with fine sandpaper or steel wool. Don't bend the rod. Reinstall. 90% of these cases fixed in 10 minutes.

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25%

Overheating (Tripped Limit Switch)

What's happening

Restricted airflow causes the heat exchanger to overheat. Limit switch opens as a safety, shutting off the burners. Blower continues until temperature drops, then everything restarts.

How to check

Air filter heavily clogged or many supply vents closed. Furnace runs longer (10-20 min) before shutting off (overheat takes time to develop).

How to fix

Replace filter immediately. Open all supply vents. Check blower wheel for dust buildup. If still happens, the limit switch itself may have failed (replace for $25-50).

Shop Limit Switches
15%

Pressure Switch Issues (Drafting Problems)

What's happening

The inducer draft fan creates negative pressure. If exhaust is partially blocked, pressure drops mid-cycle and the pressure switch opens — safety shutdown. Common in winter with snow blocking vent terminations.

How to check

High-efficiency furnaces (PVC vent pipes). Check vent terminations outside for snow, ice, bird nests, or debris. LED code often shows pressure switch fault.

How to fix

Clear vent terminations. Check inside the unit for cracked silicone hoses or blocked pressure ports. Replace pressure switch if it's bad ($30-80 part).

Shop Pressure Switches
12%

Flame Rollout Switch Tripped

What's happening

Flame rollout switches detect flames that escape the combustion chamber (rolling out toward the front of the unit). Trip = system shuts down for safety. DANGEROUS — indicates serious combustion or heat exchanger problem.

How to check

LED diagnostic code points to flame rollout. Visual inspection shows soot or scorch marks on the front of the burner assembly. Manual reset button may be visible on the switch.

How to fix

DON'T just reset and keep running — find the cause first. Could be cracked heat exchanger (full system replacement), clogged exhaust, or burner problems. Call a tech immediately.

8%

Low Gas Pressure

What's happening

Inadequate gas pressure causes the flame to be weak or unstable. Flame sensor can't detect properly, or flame rolls out. System shuts down.

How to check

Other gas appliances (water heater, stove) also seem weak. Pilot or burner flames look small. Most common during peak demand (winter mornings when everyone's water heater is running).

How to fix

Licensed gas tech measures pressure at the furnace inlet. May need utility service call for line pressure issues, or regulator adjustment at the furnace.

Shop Gas Valves
5%

Control Board Glitches

What's happening

Failing control board issues false shutdown commands or misreads sensor inputs. Often shows erratic blink codes or no LED at all.

How to check

Visual inspection: burn marks on board, swollen capacitors, loose connections. Diagnostic codes don't match observed symptoms.

How to fix

Replace control board. Match manufacturer part number. $150-400 part.

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Flame SensorsLimit SwitchesPressure SwitchesGas ValvesControl Boards

02 · Frequently Asked Questions

What's the difference between short cycling and shutting off?
Short cycling = quick on/off cycles every 2-5 minutes. Furnace shutting off = runs for 5-20 minutes, then shuts down for an extended period. Different diagnosis: short cycling is usually overheating or oversizing; full shutoff is usually flame sensor or safety switch.
How do I read my furnace blink code?
Look through the inspection window for a flashing LED. Count flashes in groups separated by pauses. Look up the code in the manual or on a label inside the furnace. Common codes: 3 flashes = pressure switch, 4 = ignition lockout, 5 = flame sensor.
Why does my furnace work for 5 minutes then quit?
Classic dirty flame sensor symptom. The burners light fine, but after a few minutes the oxidized sensor can no longer detect the flame and the gas valve closes for safety. Clean the flame sensor with fine sandpaper — fix is free if you DIY.
Is it safe to keep restarting my furnace when it shuts off?
Generally OK for flame sensor or overheat issues. NOT OK if a flame rollout switch is tripping — that indicates combustion gases are escaping, which is a fire and CO hazard. Find the cause before repeatedly resetting.

03 ·Related Symptoms

Furnace Short CyclingFurnace Not IgnitingFurnace Blowing Cold Air
⚠ Safety notice: This guide is for informational purposes only. HVAC systems involve high-voltage electricity, refrigerants under pressure, and natural gas — all of which can cause serious injury, death, or property damage. Refrigerant handling requires EPA Section 608 certification by federal law. When in doubt, contact a licensed HVAC professional.
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