Furnace grounding and reversed polarity codes

Several manufacturers publish a code specifically for reversed line voltage polarity or a missing ground, because flame sensing physically cannot work without a good neutral and ground reference.

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.

Why grounding and polarity matter to a furnace

Flame rectification measures a current of a few microamps flowing from the flame, through the burner and the furnace chassis, back to the control. That circuit is referenced to neutral and to the chassis ground. If line and neutral are swapped at the outlet or in the furnace junction box, or if the chassis is not properly bonded, the control cannot read the flame signal even when the burner is lit perfectly.

Why one wiring fault produces several different codes

The result is a furnace that lights and then shuts down a few seconds later, repeatedly, with no fault in any part it names — which is why several brands report grounding and polarity in the same code as an igniter or flame sensing fault. Trane publishes a specific measurement for this: under two volts between line neutral and 24 volt common. Anything higher points at the ground and neutral path rather than at any component.

Every grounding and line polarity code on this site

16 codes across 10 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	2 flashes	Improper earth ground
Carrier	LED rapid flashing	Line voltage polarity reversed
Goodman	8 flashes	Igniter circuit fault, most often caused by poor grounding
Goodman	rapid flashing	Reversed 115 volt polarity, a grounding problem, or an open igniter circuit
Goodman	E7	Igniter fault or improper grounding
Goodman	EA	Reversed 115 volt polarity
Goodman	9 red flashes	Grounding problem or reversed polarity
Lennox	Alert code 117	Poor earth ground on the reporting unit
Lennox	E111	Line voltage polarity reversed
Lennox	E112	Ground not detected — system shuts down
Lennox	E117	Poor ground detected — warning only
Trane	6 flashes	115 volt AC power reversed, or poor grounding
Trane	E6.1	Line voltage polarity reversed
Trane	E6.2	Bad grounding
York	9 red flashes	Grounding, reversed polarity, or reversed transformer leads

Before you work on it

Confirming polarity and ground means measuring live line voltage. This is electrical work, and in most places correcting reversed polarity at an outlet or panel is licensed work.

What the fixes cost

This family is nearly always a labor bill, not a parts bill. The fault lives in wiring — a swapped line and neutral, a broken ground path — so the typical spend is the repair guide's $75–150 diagnostic plus modest electrical correction, often by an electrician rather than an HVAC company. The money risk runs the other way: because a bad ground starves flame rectification, these faults impersonate flame-sensor trouble, and the wasteful outcome is paying for sensor cleanings at $80–200 a visit that keep not fixing it. A grounding code on the board is the cheap early answer; take it at its word before buying parts.

Do it yourself or call someone?

The observation layer is yours: did the code follow electrical work in the house, a new receptacle, a generator hookup, a replaced service switch? A furnace that ran cleanly for years rarely invents a polarity fault from nothing, and that history is worth stating when you call. The verification layer is not yours — confirming polarity and ground quality means measuring live line voltage, and correcting them at an outlet or panel is licensed work in most places. This is the rare furnace fault where the right professional may be an electrician first and an HVAC technician second.

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.