Furnace Blowing Cold Air — Flame Sensor, Limit Switch, Rollout Switch

Common Symptoms

• Blower runs but air from vents is cold or room temperature
• Furnace fires briefly then shuts off and the blower continues to run cold
• Furnace ran fine but after a very cold spell it now blows cold air
• Error codes for limit switch or rollout switch fault
• Burning smell followed by cold air from vents

Most Likely Causes

1. 1

   Dirty Flame Sensor (Most Common)

   A coated flame sensor causes the furnace to light for 2–3 seconds and then extinguish. The blower continues to run (on a timer) even though there's no flame — resulting in cold air from the vents. Clean the sensor with fine steel wool. See our gas furnace not igniting guide for the exact procedure.

2. 2

   High-Limit Switch Tripped

   The high-limit switch is a safety device that shuts off the gas valve when the heat exchanger or plenum overheats. Once the limit trips, the gas valve closes and the system blows cold air until the limit resets. Common causes: clogged air filter, blocked vents, or a failed blower motor.

3. 3

   Rollout Switch Tripped

   The rollout switch protects against flame rollout — when burner flames extend outside the combustion chamber due to a cracked heat exchanger, blocked flue, or pressure imbalance. A tripped rollout switch must be manually reset, but the cause must be identified first. Flame rollout is a serious condition indicating potential heat exchanger failure.

4. 4

   Short-Cycling on Overheating

   If the high-limit switch trips repeatedly — furnace fires, blows for 10–15 minutes, then goes cold, then fires again — the heat exchanger is overheating due to restricted airflow. Change the air filter first. Check that all registers are open and the blower runs at full speed.

5. 5

   Gas Valve Shutting Off Prematurely

   A gas valve with a failing solenoid may drop out before the full heat cycle completes, leaving the blower running cold. If the flame sensor tests clean and the limit switches test good, check for 24V at the gas valve coil throughout the entire cycle.

Quick DIY Checks

1. 1Change the air filter immediately. A clogged filter is the single most common cause of limit switch trips and short-cycling. Install a new filter before doing anything else.
2. 2Check for a manual reset button on the high-limit switch and rollout switch. These are small round buttons, often red, on the switch bodies inside the burner area. If a rollout switch has tripped, understand why before pressing reset — flame rollout means fire was burning outside the combustion chamber.
3. 3Clean the flame sensor. See our gas furnace not igniting guide for the exact steps — rub the metal rod with 0000 steel wool, reinstall, and confirm the flame holds for more than 30 seconds.

1. 4Observe the blower motor when the furnace fires. The blower should start within 30–60 seconds and run at a consistent speed. If it seems slow (reduced airflow), the blower capacitor may have failed — causing the motor to overheat and trip the limit switch.
2. 5Confirm all supply and return vents are fully open throughout the house. Closed vents increase static pressure, reduce airflow, and are a leading cause of limit switch trips in older systems.

Repair vs Replace

Recommended Tools & Parts

• Universal Furnace High Limit Switch (190°F)

  Replacement high-limit temperature switch for gas furnaces. Replace when switch tests open at room temperature or won't reset. Match temperature rating to original.

  $15–$35

  Buy on Amazon →

• Furnace Rollout Switch (250°F Universal)

  Manual-reset rollout switch for gas furnaces. Replace when switch body is cracked or switch won't reset after the cause of rollout is corrected.

  $12–$25

  Buy on Amazon →

• Furnace Blower Motor Capacitor

  Run capacitor for furnace blower motor. Replace when blower runs slowly or hums without starting — slow blower causes limit switch trips.

  $8–$20

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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