A blinking light on a furnace’s control board is often the furnace telling you what it detected. Many furnaces flash a short pattern that stands for a fault code, and you can usually read it without any tools. Knowing what the code says also makes for a better conversation if you do need to call for service.
One caution before you start: blink patterns and their meanings are set by each manufacturer, so a two-flash code on one brand can mean something different on another. Check your own furnace’s legend or manual rather than a code chart from somewhere else online.
The short version is to find the light or display, count the flashes or read the code, and match it to the legend on the unit or in its manual. Some codes point to something you can check yourself, such as the air filter. Others, like a flame rollout fault, mean the equipment should stay off until a technician has looked at it.
Where to Find the Error Code on Your Furnace
On many gas furnaces, a small diagnostic light sits behind a window in the blower door or beside the control board, and some newer models show a code on a small display instead. Where it sits varies by model, and your owner’s manual shows the location for yours.
Most codes are a count of flashes, a pause, and then a second count. Two quick flashes, a pause, and three more flashes would be read as a 2-3 pattern, though what that pattern means depends on the manufacturer. Count each group more than once, since it is easy to miscount the first time. A display works the same way, except that you read letters or numbers instead of counting flashes.
Look for a label listing the fault codes on the inside of the access door. Manufacturers commonly put one there, and the owner’s manual lists the codes too. If you have to remove a door to read the label, follow the manual’s instructions for doing it, and put the door back before the unit is restarted. A quick photo of the label with your phone is handy if you end up calling for service.
What Common Furnace Fault Codes Tend to Mean
Brand to brand, the codes differ, but many gas furnace manuals sort faults into a few families. Knowing the family helps you decide whether something is worth checking yourself or whether to call for service right away.
Ignition and Flame Sensing Faults
Ignition faults generally mean the furnace tried to light its burners, could not confirm a flame, and shut the gas off again. A dirty or worn flame sensor, the small rod that tells the control board a flame is present, is one possible cause. Weak ignition parts and gas supply problems are other possible causes, and there can be more. Because gas is involved, this is a code to hand to a technician rather than work around.
Pressure Switch Faults
With a pressure switch code, the control board is not getting the signal it expects from a device that checks the venting. The cause could be a blocked or damaged vent, a failed switch, or another part of the venting system, and telling them apart takes testing.
High-Limit and Overheating Faults
A high-limit code generally means the unit shut itself off because it got hotter than it should. Restricted airflow is one possible cause, and a dirty air filter is the easiest one to check. Blocked vents and a failing blower motor are other possibilities, so a code that repeats deserves a professional look. Our guide to furnace overheating covers more of the causes.
Flame Rollout Faults
A flame rollout code is one of the more serious ones. Do not reset it and carry on. Have a heating technician find the cause before the furnace runs again.
Using Your Manual to Decode the Pattern
The label inside the door covers the basics, but the owner’s manual is the fuller reference. If you no longer have a paper copy, most manufacturers post manuals online, and a search for your model number usually finds them.
Model and serial numbers are on a rating plate on the furnace. Write both down before you search, since manuals are organized by model. Most manuals list each fault code with a likely cause, which is the information a technician will want to hear from you.
Once you have the manual open, find the troubleshooting or diagnostic section and match your count to the table. Four flashes, a pause, and then two flashes, for example, would be a 4-2 pattern to look up. Write down the description before you close the manual, and mention it if you call for furnace repair in Newtown.
Now and then the display shows a code the manual does not list, for example after a control board has been replaced. The replacement board may have its own documentation, so check it for a part number and search for that. If the code still does not match anything, call a technician instead of guessing.
Codes You Can Reasonably Check Yourself
Not every code needs a service call, but the list of homeowner checks is short. If your unit shows a high-limit or overheating code, check the air filter first. ENERGY STAR advises homeowners to inspect, clean, or change air filters once a month in a furnace, and its checklist states: “A dirty filter can increase energy costs and damage your equipment, leading to early failure.” Hold the filter up to a light, and if you cannot see light through it, replace it with the size and type the manual calls for.
A lockout is another common situation. Some furnaces lock out after several failed ignition attempts and stay off until they are reset. Your manual explains how to reset yours, and the procedure varies by model. If the furnace then lights and runs normally, the fault may have been a one-time event. If the same code comes back, resetting again is not a fix, because the cause is still there.
Condensing (high-efficiency) furnaces drain water through a condensate line. If you see water on the floor near one, tell your technician rather than taking drain lines apart yourself while the furnace has power.
When to Stop and Call a Professional
Some codes are not ones to reset your way through. Anything involving gas, burners, venting, or electrical components belongs to a qualified professional.
If an ignition or flame sensor code returns more than once in a short stretch, resetting is not a fix. The cause could be the sensor, an ignition part, or the gas supply, and a technician can sort out which. The same goes for a pressure switch code that keeps returning, since the cause is somewhere in the venting, the draft components, or the switch itself. Do not try to bypass a safety switch to keep the furnace running.
If a CO alarm sounds, get everyone, pets included, outside and call 911. Carbon monoxide is a colorless, odorless gas that interferes with the delivery of oxygen throughout the body. A unit that keeps faulting is better inspected than reset again and again.
Faults with the control board or with communication between components are not homeowner repairs either. A technician can test whether the board itself has failed or whether another component is the cause. If a fault appears at a bad time, Service First offers emergency furnace repair in Bucks County.
Getting Help With a Furnace Fault Code in Bucks County
Service First has served Bucks County homeowners since 2009 and is a Trane Comfort Specialist. If an older unit keeps throwing codes for different parts, it may be time to weigh repair against replacement, and a technician can help you compare the two. A routine HVAC maintenance visit is another way to catch problems before they show up as a code, and ENERGY STAR recommends annual pre-season check-ups by a contractor for heating and cooling systems.
Frequently Asked Questions
Where do I find my furnace's error code?
On many gas furnaces, a small diagnostic light sits behind a window in the blower door or beside the control board, and some newer models show a code on a small display instead. Where it sits varies by model, and your owner’s manual shows the location for yours. A label listing the fault codes is often on the inside of the access door, and the owner’s manual lists the codes too. Do not remove panels to look for it unless your manual tells you to. If you cannot read the code without opening the furnace, call a technician.
Do all furnace brands use the same blink codes?
No. Blink counts and their meanings are set by each manufacturer, and sometimes vary by model line within the same brand. Always check the legend printed inside your specific furnace’s access panel or its owner’s manual rather than assuming a code from another brand applies.
What does a flame rollout code mean?
A flame rollout code is one of the more serious ones. What triggers it and how it is labeled depends on the manufacturer, so check your manual for what it means on your furnace. Do not reset it and carry on. Have a heating technician find the cause before the furnace runs again.
What should I check first with a high-limit or overheating code?
Check the air filter first. A clogged air filter can adversely impact airflow and cause your furnace to overheat. A furnace limit switch is a built-in safety feature in your furnace. It’s responsible for keeping your furnace from overheating, which could result in damage to your furnace and your home. Hold the filter up to a light, and if you cannot see light through it, replace it with the size and type your manual calls for. Make sure that furniture and curtains are not blocking registers or return air vents. Do not try to bypass or reset the limit switch. If the code comes back, call a technician.
What if my code isn't in my manual?
It can happen, for example after a control board has been replaced. The replacement board may have its own documentation, so check it for a part number and search for that. If the code still does not match anything, call a technician instead of guessing or resetting the furnace again and again. Codes and their meanings are set by each manufacturer, so a chart from another brand will not tell you what yours means.
Is it safe to keep resetting my furnace when a code appears?
A single reset after an isolated fault is generally fine. If the same code returns within a day or two, resetting it again without addressing the cause is not a fix, and for a flame rollout fault you should not reset the furnace at all before a technician inspects it.