Furnace Blinking Light: How to Read Your Furnace's Error Codes

Most modern furnaces have a small light on the control board, visible through a window in the lower door. When the furnace has a problem, that light blinks in a pattern. It's the furnace telling you what's wrong, and reading it before you call can save time and sometimes a service visit.

Here's how to read the code, what the most common codes usually point to, and when to stop and call for help.

Read the light before you open anything

This is the most important tip. Many furnaces have a safety switch on the lower door that cuts power when the door comes off. When the power cuts, the furnace can lose the current code. So look through the little window first.

• Watch the light for a full minute. Most codes repeat with a pause in between.
• Count the flashes in one group. Some codes combine long and short flashes, like one long followed by three short.
• Note whether the light is steady, blinking slowly, blinking fast or off.
• Write it down or take a short video on your phone.
• Find the code chart. It's usually a label on the inside of the furnace door or on the side of the cabinet. Many manufacturers also post code charts for each model online.

What the light usually means when nothing's wrong

On many furnaces, a steady glow or a slow, regular blink means normal operation, and some boards blink differently while the furnace is heating. If the house is warm and the light is doing what the chart calls normal, there's nothing to do. A light that's completely off can mean the furnace has no power: check the service switch, which looks like a light switch on or near the furnace, and the breaker.

Common codes and what they usually point to

Every manufacturer numbers its codes differently, so always go by the chart on your furnace. But most codes fall into the same handful of categories.

• Pressure switch problem. The inducer runs but the switch doesn't confirm draft. Usually a blocked vent pipe, a clogged condensate drain or a hose problem. Our post on the furnace pressure switch covers it.
• Limit switch open. The furnace overheated and shut the burners off. A dirty filter or blocked returns are the most common cause.
• Ignition failure or lockout. The furnace tried to light several times and gave up. Often a dirty flame sensor or a weak igniter. See the furnace flame sensor.
• Flame sensed with the gas valve closed. A serious fault that points to a gas valve problem. Turn the furnace off and call.
• Reversed polarity or grounding problem. An electrical wiring issue, sometimes after other electrical work in the house.
• Blown fuse or low voltage. Often a short in the thermostat wiring or a failed component.
• Blower or motor fault. Common on variable-speed furnaces when the blower motor or its module has a problem.

What you can safely try

• Replace a dirty filter. That alone clears a lot of limit codes. Our guide on how often to change a furnace filter explains sizes and MERV ratings.
• Check the white intake and exhaust pipes outside for snow, ice or debris, especially after a storm.
• Check that the condensate drain hose isn't kinked and the condensate pump isn't full.
• Make sure the thermostat is set to heat and has fresh batteries.
• Reset the furnace once by turning off the service switch for a minute and back on. If the same code comes back, stop resetting and call.

Don't open the burner compartment, touch the gas valve or jump any switches. And if you smell gas, hear a carbon monoxide alarm or see scorch marks or melted wiring, get out first and call 911 or your gas utility from outside. Our post on why your CO detector is beeping explains the alarm patterns.

Why the code helps even if you call

When you call for service, tell us the code, the pattern and the furnace brand if you know it. It narrows down what the technician should bring and look at first. It also helps us tell you over the phone whether it's something you can safely handle, like a filter or a snowed-in vent pipe, or something that needs a visit.

If the code keeps coming back after a repair, ask whether the underlying cause was fixed. A limit code that returns every week usually means an airflow problem, not a bad switch. A recurring ignition code can mean a combustion problem rather than just a dirty sensor.

Codes on your thermostat

Some newer systems also show alerts on the thermostat itself. Communicating systems, where the furnace, outdoor unit and thermostat talk to each other, can display a fault message and keep a history of recent codes. Many smart thermostats send a phone alert when the heat isn't keeping up, even though they can't read the furnace's own code. If your thermostat shows an alert, write it down along with the blinking pattern at the furnace. The two together often tell the technician exactly where to look.

If the thermostat screen is blank, that's a clue too. A blank thermostat on a furnace that also has no status light usually means the furnace has lost power or blown a low-voltage fuse.

Bryant and other brands

As a Bryant Factory Authorized Dealer, we see a lot of Bryant furnaces, but we repair every make and model. Each brand, and often each model, has its own code chart, which is why the label on your furnace is the best guide. If the label is missing or faded, we can look it up from the model number on the furnace's rating plate.

Blinking light and no heat?

Write down the code, try the safe checks above and, if the furnace still won't run, schedule a furnace repair or call Heating & Cooling Two at 763-428-3677. Office hours are Monday through Friday, 7:30 a.m. to 4:30 p.m., and after-hours emergency service is available. For the next steps in order, see our furnace troubleshooting checklist. An annual furnace tune-up catches most of the problems behind these codes before they leave you without heat.