Furnace voltage and fuse codes by brand

A blown control-board fuse is almost never the fault. It is the evidence of one, and fitting a new fuse without finding what opened the old one simply blows the new one.

Some of the faults listed here are gas-safety faults. Those ones involve the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes, and each carries this warning on its own page. Do not keep resetting a furnace that reports one of them in order to hold the heat on. Have it looked at by a licensed technician, and make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on. The conditions that mean stop now.

How a furnace is powered

A furnace runs on two supplies at once. Line voltage, 115 V in most homes, drives the motors, the igniter and the transformer. The transformer steps that down to 24 V for the control circuit — the thermostat, the switches, the gas valve coil — and a small fuse on the control board, usually rated three or five amps, protects that low-voltage side. Codes in this group report either that a supply is outside the range the control will run on, or that the fuse protecting the low-voltage side has opened.

Why one supply produces several different codes

Those two are diagnosed in opposite directions. A supply complaint — low line voltage, high line voltage, a frequency outside the accepted band — is usually about what is arriving at the furnace rather than the furnace itself: a brownout, an undersized or shared circuit, a long run of thin cable, or a generator whose output is not a clean 60 Hz. That last one catches people out during an outage, because a furnace that will not run on generator power is often reporting exactly this and is not damaged. A blown low-voltage fuse is the opposite: it is a symptom, and the fault is a short somewhere on the 24 V side. Pinched thermostat cable, a condensate switch wire rubbed through against sheet metal, and a scorched wire near the burner compartment are the usual places. Note also that several manufacturers report a blown fuse and an open rollout switch on the same code, so on those boards the same signal means either — and one of the two is a safety fault.

Every voltage and fuse code on this site

19 codes across 11 control families. The meaning is the manufacturer's, from the document listed at the foot of each code page.

Brand	Code	What it reports
Armstrong	1 flash	Reverse line voltage polarity
Armstrong	13 flashes	Low line voltage
Carrier	LED rapid flashing	Line voltage polarity reversed
Carrier	Status code 24	Secondary voltage fuse is open
Goodman	6 flashes	Rollout limit open, or control module fuse blown
Goodman	b6	Blower motor over or under voltage trip, or over temperature trip
Goodman	E5	Low voltage fuse is open
Goodman	5 red flashes	Open rollout switch, or open fuse detected
Lennox	Alert code 116	High 24 volt secondary voltage
Lennox	Alert code 370	Interlock switch open — no 24 volts at the DS terminal
Lennox	E110	Low line voltage
Lennox	E111	Line voltage polarity reversed
Lennox	E113	High line voltage
Lennox	E114	Line voltage frequency out of range
Lennox	E115	Low 24 volt supply
Trane	9 flashes	Ignitor circuit problem, or low voltage between neutral and 24 volt common
Trane	E6.1	Line voltage polarity reversed
Trane	E12	Open fuse
York	Rapid red flashing	Twinning error or incorrect 24 volt phasing

Before you work on it

Never fit a fuse of a higher rating than the one that opened. The fuse is sized to the wiring it protects, and a larger one lets a shorted low-voltage circuit keep drawing current through cable that cannot carry it. If a correctly rated fuse opens again, the short is still there and needs finding rather than working around.

What the fixes cost

The signature repair here is the cheapest part on the furnace wrapped in a labor bill: the board's low-voltage fuse, $5–150 fitted per the control board cost guide, where the cost is finding the short that opened it — usually a pinched or chafed thermostat wire. That same guide's warning is this page's economics in one line: a popped fuse impersonates a dead board, and boards run $400–900 installed. The supply-voltage codes point the other direction entirely, at the house wiring or the utility, where the furnace-side cost is a $75–150 confirmation and the fix belongs to an electrician. Between those poles, very little on this page should end in an expensive furnace part.

Do it yourself or call someone?

Yours: the breaker, the service switch, and the blower door panel whose interlock cuts all power when it sits loose — the three no-tools checks that resurrect many dark furnaces. Also yours: refusing the wrong fix, which this page's warning already covers — never a larger fuse. Not yours: the fuse replacement itself, because the short that opened it is still in the low-voltage wiring and finding it is meter work; and anything inside the electrical panel. If a dead-board quote arrives without the word fuse having been said, say it yourself.

Other parts

Sources

Every code meaning above comes from the manufacturer document behind its code page:

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.