Trane and American Standard furnace 6 flashes: polarity, ground or igniter

Short answer On Trane and American Standard single-LED boards (*UC1/*DC1, *UX1/*DX1, *UE1/*DE1, CUX1/CDX1, CUB1/CDB1), 6 flashes mean 115 volt AC power reversed, poor grounding or system voltage too low; other series differ, see the table below. This is wiring work for a licensed technician. See what to check

Furnace control LED or display

6 flashes

“115 volt AC power reversed, poor grounding or system voltage too low”

Technician

Printed on 14 model series · 7 documents

*UH1-H, *DH1-H, *UC1, *DC1, *UX1, *DX1, *UD1-H, *DD1-H, *UE1, *DE1, CUX1, CDX1, CUB1, CDB1 · 18-CD33D1-11-EN p. 39; 18-CD19D5-35-EN p. 40; 18-CD19D6-34 p. 39; 18-CD21D1-15-EN p. 24; 18-CD19D4-28-EN p. 21; 41-5016-12 p. 46; 41-5010-25 p. 28

The lamp blinks 6 times, then pauses. It shows the count only, not the color; the timing on your board may differ.

What it means

Six flashes name reversed polarity, the ground or an igniter fault. Most Trane lists with a 6-flash row name reversed polarity; most add the ground, some add an igniter (Triac) fault, and some a system voltage that is too low. The wording depends on the series and the board:

Table	Series or board	6 flashes, as printed	Source
Single LED	*UC1/*DC1, *UX1/*DX1, *UE1/*DE1, the H models of the *UD1/*DD1 and *UH1/*DH1, CUX1/CDX1, CUB1/CDB1	“115 volt AC power reversed, poor grounding or system voltage too low”	18-CD19D5-35-EN, p. 40; 18-CD19D6-34, p. 39; 18-CD19D4-28-EN, p. 21; 18-CD21D1-15-EN, p. 24; 18-CD33D1-11-EN, p. 39; 41-5016-12, p. 46; 41-5010-25, p. 28
Single LED	*UD1/*DD1	“115 Volt AC power reversed or Poor Grounding or system voltage too low”	18-CD19D7-29-EN, p. 22
Single LED	*UH1/*DH1	“115 Volt AC power reversed or Poor Grounding”	18-CD29D1-18-EN, p. 40
Red LED	*UX2/*DX2, *UD2	“115 volt AC power reversed, ignitor (Triac) fault, poor grounding or system voltage too low”	18-CD19D8-20, p. 37; 18-CD25D1-6, p. 41; 18-CD24D1-6, p. 28
Red LED	*UH2/*DH2	“LINE REVERSE POLARITY”	18-CD26D1-23-EN, p. 44
Board guides that list no furnace models	CNT04829, CNT05457	“Reversed polarity, or Igniter (Triac) Fault”	18-CH42D1-2, p. 6; 18-CH50D1-2, p. 7
Board guide that lists no furnace models	CNT05141	“Reversed polarity, or Igniter (Triac) Fault / Ground fault”	18-CH46D1-2, p. 6
Replacement control kit 17399	*UD2/*DD2 models in the kit guide	“115 VOLT AC POWER REVERSED OR IGNITER FAULT”	18-CH76D1-1A-EN, p. 2
Replacement control kits 18109, 18110, 20350	the models listed in each kit guide	“115 VOLT AC POWER REVERSED OR IMPROPER GROUND”	18-CH92D1-1A-EN, p. 2; 18-CH93D1-1A-EN, p. 2; 18-CH111D1-1A-EN, p. 2
Alert tables	*UH3/*DH3, *UHM/*DHM and the three boards above	POLARITY ERR (alert code 33, “Voltage reverse polarity”), GND FAULT (alert code 88, “Occurs when proper earth ground is not detected.”), IGNITER ERR and TRIAC ERR (alert code 10, group “Ignition Means Fault”)	18-CD23D1-9, p. 51; 18-CD30D1-21-EN, p. 55; 18-CH42D1-2, p. 7; 18-CH46D1-2, p. 7; 18-CH50D1-2, p. 8

The S9X troubleshooting charts define the two faults for their display codes: a polarity fault means the incoming high voltage wiring is reversed, and a ground fault means the incoming or chassis ground connection is not sensed (FNR-SVX002B-EN, p. 108). On the flash boards, the count alone does not tell which of the listed faults it is; on the communicating furnaces the alert name does (18-CD30D1-21-EN, p. 55).

Check these first

The owner’s information (32-5083-1C-EN) has no code list and no step for 6 flashes, and it says nothing about polarity or grounding. It warns that bodily injury can result from high voltage electrical components and that during installation and servicing the electrical supply must be disconnected (32-5083-1C-EN, p. 3).

Its troubleshooting table goes by symptom and does not mention codes. The rows an owner can act on, in the table’s order:

1. Check that the thermostat is set correctly (32-5083-1C-EN, p. 3).
2. For a blown fuse or tripped circuit breaker: “Replace or reset protective device or call for servicer.” (32-5083-1C-EN, p. 3).
3. If the main gas line is turned off: “Have gas company check.” (32-5083-1C-EN, p. 3).
4. Make sure the blower door is closed securely; the table says this restores power to the blower and gas valve (32-5083-1C-EN, p. 3).
5. For heat that is not enough while the blower runs all the time: clean or replace dirty air filters, and make sure the supply and return registers are open with nothing blocking the air (32-5083-1C-EN, p. 3).
6. For no heat while the vent motor runs: make sure the vent terminals are not blocked; a restricted or plugged condensate drain is for a servicer (32-5083-1C-EN, p. 3).

If the furnace still will not operate, turn off the gas to the unit and call your service technician or gas supplier (32-5083-1C-EN, p. 2). The steps turn off all electric power before the front panel comes off (32-5083-1C-EN, p. 3): turning off the gas.

The list is the same on every code page. None of its rows is about the wiring that the 6-flash rows name, and these pages do not present it as a fix. Do not touch the igniter; the manual says it is extremely hot (32-5083-1C-EN, p. 2).

When to call a licensed technician

• Checking and correcting the 115-volt supply wiring, its polarity and the furnace ground. This is electrical work at the furnace and its supply, for a licensed technician. The flash tables give no corrective actions.
• An igniter (Triac) fault, on the boards whose rows name it (18-CD19D8-20, p. 37; 18-CH42D1-2, p. 6).
• A system voltage that is too low, on the boards whose rows name it (18-CD19D5-35-EN, p. 40).
• 6 flashes keep coming back. The owner’s information gives no step for codes; these pages send it to the technician.

For its own display codes, the S9VM table lists these causes: line hot and line neutral reversed for E6.1; for E6.2, the furnace ground not connected to earth ground, chassis grounds not connected or poorly connected, and the incoming power supply not grounded to the main electrical panel (FNR-SVX006B-EN, p. 105). That table is for the S9VM only.

Notes for different models

Series	Code	Wording	Source
S8B1, S8X1, S8X2, S9B1, S9X1, S9X2, S8V2-C, S9V2-VC (Link kit)	E6.1, E6.2	“Voltage reversed polarity”; “Bad grounding”	S8XB-SVX001-1D-EN, p. 58; FNR-SVX002B-EN, p. 98; FNR-SVX003A-EN, p. 59; FNR-SVN001A-EN, p. 45
S8B1, S8X1, S8X2, S9B1, S9X1, S9X2	E6.3	“(1) Igniter relay fails (2) Igniter open”	S8XB-SVX001-1D-EN, p. 59; FNR-SVX002B-EN, p. 98
L8V1	E6.1, E6.2	“Hot & neutral reversed”; “Ground not detected”	FNR-SVX007A-EN, p. 43
S9VM	E6.1, E6.2	“Incoming High Voltage Reversed Polarity”; “Insufficient / Bad Grounding”	FNR-SVX006B-EN, p. 105
*UX1/*DX1, the H models of the *UD1/*DD1 and *UH1/*DH1	9 flashes	a check of the igniter circuit and a voltage, ending “[possible grounding problem]”	18-CD19D6-34, p. 39; 18-CD21D1-15-EN, p. 24; 18-CD33D1-11-EN, p. 39

On those three series the 9 flashes row also mentions a possible grounding problem. The documents do not translate flash counts into display codes.

Related Trane and American Standard codes

• 9 flashes “Ignitor Relay Fault”

See also

• Heat pump vs gas calculator
• Furnace and HVAC tax credits for 2025 and 2026: 25C ended after 2025
• Form 5695 for 2025 and 2026: QMID, lines, carryforward
• How a gas furnace lights, and how the control board blinks a failure code
• Home maintenance schedule from the manuals: monthly to yearly
• Winter home maintenance checklist: furnace, boiler, water heater
• Water heater or furnace closet: clearance and air requirements

Other gas furnaces: Carrier, Goodman, Lennox, Rheem furnaces, York, Coleman and Luxaire

All Trane and American Standard codes

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