If your furnace stops heating properly and a light on the control board begins flashing, the furnace may be displaying an error code.
These codes are designed to help identify where the system detected a problem. The important thing to remember is that furnace error codes are not universal. A code that means one thing on a Carrier furnace may mean something completely different on a Lennox, Trane, Goodman, Bryant, Rheem or another brand. The code is a diagnostic clue, not necessarily a final diagnosis.
How do furnace error codes work?
Many modern furnaces have a small LED light on the control board.
The light may:
Stay solid
Blink slowly
Blink rapidly
Flash a certain number of times
Alternate between different patterns
The furnace manufacturer assigns a meaning to each pattern. For example, a certain number of flashes may indicate an ignition failure, pressure switch fault or high-limit problem. The exact meaning depends on the furnace model.
Where do I find the furnace error code chart?
Many furnaces have a diagnostic chart printed on a label inside the access panel.
The chart may list:
The flash pattern
The corresponding fault
Possible causes
Whether the furnace will retry automatically
Whether the system has entered lockout
The owner’s manual may also include the code list. If you can safely access the furnace panel, look for a label near the control board or on the inside of the door.
What does a furnace ignition error mean?
An ignition-related code usually means the furnace tried to light the burners but could not complete the ignition sequence properly.
Possible causes include:
A failed igniter
A dirty flame sensor
A gas valve problem
A gas supply issue
Dirty burners
Wiring problems
A control board problem
The code tells you that the furnace detected an ignition failure, but it may not tell you which component caused it.
What does a pressure switch error mean?
A pressure switch code usually means the furnace did not detect the proper venting conditions.
Possible causes include:
A blocked intake or exhaust pipe
A weak inducer motor
Damaged pressure-switch tubing
A clogged condensate drain
A vent restriction
A faulty pressure switch
A pressure switch fault does not automatically mean the pressure switch itself is bad. The real problem may be elsewhere in the venting system.
What does a high-limit error mean?
A high-limit fault usually means the furnace became too hot. The high-limit switch is a safety control that shuts the burners off when furnace temperature becomes excessive.
Common causes include:
A dirty furnace filter
Closed or blocked registers
A dirty blower wheel
A weak blower motor
Restricted ductwork
An overly restrictive filter
Other airflow problems
If the furnace repeatedly trips the high-limit switch, the cause should be corrected rather than simply resetting the system.
What does a flame sensor error mean?
A furnace may display a flame-related fault when it cannot verify that the burners are producing a proper flame. A dirty or failing flame sensor is one possible cause. You may notice the burners light and then go out after only a few seconds.
Other causes can include:
Poor electrical grounding
Loose wiring
Burner problems
Gas pressure issues
Control board faults
The flame sensor should be tested rather than automatically replaced.
What does a rollout switch error mean?
A rollout switch is a safety device designed to detect abnormal heat or flame movement near the burner area. If a rollout switch trips, the furnace should be inspected.
Possible causes can involve:
Burner problems
Venting problems
Combustion issues
Heat exchanger problems
Other unsafe operating conditions
A rollout switch should never be bypassed.
What does a lockout code mean?
A lockout code usually means the furnace experienced repeated failed startup or safety events and stopped attempting to run.
This can happen after:
Repeated ignition failures
Repeated flame loss
Pressure switch faults
Overheating
Other safety-control problems
Some furnaces reset themselves after a waiting period. Others may require the power to be cycled after the underlying problem has been corrected. If the furnace repeatedly goes into lockout, resetting it is not a long-term repair.
What does a blower motor error mean?
Some furnaces can detect problems with the blower motor or blower control system.
Possible causes include:
A failing blower motor
A failed motor control module
Wiring problems
Control board problems
Restricted airflow
A blower problem can affect both heating performance and furnace temperature.
What does a polarity or grounding error mean?
Some furnaces monitor electrical polarity and grounding. If the furnace detects improper wiring or grounding, it may display an error code or refuse to operate properly.
These problems may involve:
Reversed electrical polarity
Poor grounding
Loose wiring
Electrical installation problems
Electrical faults should be corrected rather than bypassed.
Can you diagnose a furnace from the error code alone?
Usually not.
An error code tells you what condition the furnace detected. It does not always tell you which part failed. For example, a pressure switch error could be caused by the switch itself, but it could also be caused by a blocked vent or weak inducer motor. An ignition code could involve the igniter, gas valve, burner assembly or flame sensor. Proper diagnosis means testing the system to find the actual cause.
Should you reset the furnace after seeing an error code?
You can sometimes reset a furnace by cycling power, but it is a good idea to record the error code first. Once the furnace is reset, the code may disappear. If the furnace starts normally and continues operating without trouble, the fault may have been temporary. If the same code keeps returning, the furnace should be inspected. Repeated resets can hide a recurring problem without fixing it.
When should you call for furnace repair?
You should have the furnace inspected if:
The same error code keeps returning
The furnace will not start
The furnace repeatedly goes into lockout
The burners light and then shut off
The furnace is overheating
The blower runs without heat
The furnace is making unusual noises
You smell gas, burning or electrical odors
If possible, write down the flash pattern or take a photo before resetting the furnace. That information can help the technician diagnose the problem more quickly.
Need help with a furnace error code?
Furnace error codes can point toward ignition, airflow, venting, blower, flame-sensing or safety-control problems, but the code itself is only part of the diagnosis.
Roberts Brothers Comfort technicians can read the furnace fault code, test the related components and determine what is causing the system to malfunction. We provide expert furnace repair service throughout Ohio’s Miami Valley region including Casstown, Covington, Fletcher, Piqua, Rosewood, Saint Paris, Sidney, Tipp City, Troy, Urbana, West Milton and all the surrounding areas.
