Furnace Troubleshooting

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Furnace Troubleshooting Guide: How to Fix the Most Common Furnace Problems
When the temperature drops in Cupertino, a malfunctioning furnace can become a major issue fast. Whether your furnace won’t turn on, is blowing cold air, or keeps shutting off, homeowners often search for “furnace troubleshooting” to diagnose the problem before calling in a professional.
At Cloud Mechanical HVAC, we help homeowners across Cupertino and the South Bay restore heat quickly and safely. This guide covers the most common furnace problems, how to troubleshoot them, and when it’s time to call an HVAC technician.

How a Furnace Works Before you start furnace troubleshooting, it helps to understand the basic cycle:

1. Thermostat signals heat demand
   
2. Furnace starts blower and inducer motor
   
3. Ignition system lights the burners
   
4. Heat exchanger warms up
   
5. Blower circulates warm air through ducts
   

If any part of this sequence fails, your furnace may refuse to heat your home.

Common Furnace Problems & How to Troubleshoot ThemHere are the most frequent heating issues we see in Cupertino homes—and how to diagnose them safely.

1. Furnace Not Turning OnA furnace that won’t start is one of the most common service calls.
Troubleshooting Steps

• Check the thermostat
  Make sure it’s set to HEAT and the temperature is above room temp.
  
• Inspect the furnace switch
  Furnaces have a switch that looks like a light switch. It must be ON.
  
• Verify the circuit breaker
  If tripped, reset it once. If it trips again, call a technician.
  
• Check the furnace door
  Many furnaces won’t run if the access panel isn’t fully closed.
  

If none of these work, the issue may involve the control board, blower motor, or ignition system.

2. Furnace Blowing Cold AirIf your furnace turns on but only blows cold air, here are the likely causes:
Possible Reasons

• Furnace overheated and shut off the burners
  
• Dirty air filter
  
• Faulty flame sensor
  
• Ignition system failure
  
• Thermostat fan set to ON instead of AUTO
  

DIY Troubleshooting

• Replace your air filter
  
• Change the thermostat fan setting to AUTO
  
• Turn the power off, wait 30 seconds, and turn it back on (resets the furnace)
  

If the furnace still blows cold air, the burners may not be staying lit—this requires professional service.

3. Furnace Keeps Shutting Off Short cycling is when the furnace starts, runs briefly, then shuts off repeatedly.
Common Causes

• Dirty or clogged air filter
  
• Blocked air vents
  
• Overheating furnace
  
• Thermostat malfunction
  
• Wrong furnace size
  
• Faulty limit switch
  

Troubleshooting

• Replace the air filter
  
• Open all vents in your home
  
• Make sure nothing blocks return air vents
  

If the limit switch is failing, the furnace will need a technician to replace it.

4. Furnace Ignitor Not WorkingIf your furnace won’t ignite, the issue often involves the ignitor.
Signs of a bad ignitor

• No glow from the ignitor
  
• Clicking but no flame
  
• Furnace tries to start but shuts down
  

DIY OptionThere is no safe homeowner fix for ignitors. They are fragile and electrically sensitive.
Call a professional for proper diagnostics and replacement.

5. Furnace Making Loud or Strange NoisesYour furnace should make a soft hum—not bangs, rattles, or screeching.
Common Noise Issues

• Banging / Booming: Delayed ignition (dangerous)
  
• Grinding: Worn blower motor bearings
  
• Screeching: Loose blower belt
  
• Clicking: Bad relay or failing ignition
  

If your furnace makes any of these noises, shut it off and call a technician.

6. Furnace Not Producing Enough HeatYour furnace may run but still fail to warm your home.
Possible Causes

• Dirty filter
  
• Duct leaks
  
• Undersized furnace
  
• Thermostat placement issues
  
• Closed or blocked vents
  

What You Can Do

• Replace the filter
  
• Open all vents
  
• Check that nothing is blocking return vents
  

If the problem persists, the furnace may need recalibration or an airflow check.

7. Burning Smell From FurnaceSometimes a burning smell is normal—like when you turn your furnace on for the first time in winter.
Normal Smells

• Light dust burning off after months of inactivity
  

Unhealthy Smells

• Electrical burning
  
• Chemical odor
  
• Rotten eggs (gas leak — leave home immediately and call the gas company!)
  

If the smell persists, turn off the furnace and schedule a service call.

When to Stop Troubleshooting and Call an HVAC TechnicianFurnaces involve gas, electricity, flame, and high heat—making them dangerous to repair without proper training.
Stop DIY troubleshooting and call a professional if:

• You smell gas
  
• The furnace ignitor or flame sensor is bad
  
• Your heat exchanger may be cracked
  
• The furnace repeatedly short cycles
  
• You see water or rust around the furnace
  
• The flame goes out constantly
  

These issues can lead to damaged equipment or deadly carbon monoxide leaks.

Prevent Furnace Problems With Annual MaintenanceMost furnace failures are preventable. Annual heating maintenance includes:

• Cleaning burners
  
• Checking heat exchanger
  
• Testing flame sensor & ignitor
  
• Inspecting blower motor
  
• Checking gas pressure
  
• Flushing condensate drain
  
• Replacing the air filter
  

Regular maintenance helps your furnace last longer, run quieter, and heat more efficiently.

Need Furnace Troubleshooting in Cupertino, CA? Call Cloud Mechanical HVACIf your furnace isn’t working properly, we can diagnose the issue fast and restore heat safely.
📞 Call Cloud Mechanical HVAC today: (408) 740-7079
📍 Serving Loyola, Mountain View, Lawrence, Sunnyvale & the South Bay
💻 Schedule service at www.hvaccupertino.com
Fast service, honest repairs, and guaranteed results.

FAQ FOR FURNACE TROUBLESHOOTINGWhy is my furnace not coming on?Check the thermostat, breaker, furnace switch, and air filter. If everything looks fine, the ignition system or control board may be failing.
Why is my furnace blowing cold air?Your furnace may be overheating, have a dirty filter, or have an ignition problem.
Why does my furnace keep shutting off?Short cycling is often caused by restricted airflow, thermostat issues, or a faulty limit switch.
Is it safe to troubleshoot my furnace myself?Basic checks are safe, but anything involving gas or ignition should be handled by a professional.