Furnace blinking light codes: what they mean

The control board is the furnace's computer. It watches every safety switch and every step of the start-up, and it uses one or more small lights to say what it sees. A blinking light is not always bad news. On many furnaces a slow, steady blink or a solid light means normal operation or standby, and only certain patterns mean a fault.

That is why the legend for your own furnace matters. The same number of flashes can mean one thing on one brand and something unrelated on another, and it can differ between models from the same manufacturer. This page explains how to read the code and what the common fault groups are. It does not list brand codes, because a wrong match sends you looking for the wrong problem.

Boards use a few common schemes:

• Counted flashes. A group of flashes, a pause, then the same group again. The number of flashes is the code.
• Two-part codes. A number of short flashes followed by a number of long flashes, read as two digits.
• Speed and state. Solid on, slow blink, fast blink or off, each with its own meaning.
• Colors or multiple lights. Red, green and amber lights, alone or in combination.
• Digit display. Newer boards show letters and numbers directly.

A light that is completely off usually means no power is reaching the board: a tripped breaker, the furnace switch, a loose door or a blown fuse.

Manufacturers print the code chart on a label, most often on the inside of the lower door panel or on the cabinet wall near the board. Read the flashes through the sight window first. Then, if the label is inside, turn the furnace off at its power switch, lift the door off, read or photograph the label, and put the door back fully seated before restoring power.

If the label is missing or faded, find the model number on the rating plate, a sticker inside the cabinet or on its side, and look up the installation manual on the manufacturer's website. Many boards also store recent fault history that a technician can retrieve even after the light has cleared.

A code names the safety circuit that stopped the furnace. It does not name the failed part. A pressure switch code can come from a blocked vent, a cracked hose, a clogged drain or a weak inducer. The technician uses the code as a starting point and tests each possibility with a meter and gauge. Give them the exact pattern you saw and the model number when you book.

A rollout code, or any code that returns right after a reset, means the furnace stays off until it is inspected. Codes on their own rarely decide repair versus replacement. The part behind the code does.