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NOTE - The G27M furnace contains electronic compo
nents that are polarity sensitive. Make sure that the fur
nace is wired correctly and is properly grounded.
CAUTION
Electrostatic discharge can affect electronic com
ponents. Take precautions during furnace instal
lation and service to protect the furnace's elec
tronic controls. Precautions will help to avoid
control exposure to electrostatic discharge by
putting the furnace, the control and the techni
cian at the same electrostatic potential. Neutral
ize electrostatic charge by touching hand and all
tools on an unpainted unit surface, such as the
gas valve or blower deck, before performing any
service procedure.
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures
a-Electronic Ignition
On a call for heat the SureLight control monitors the
combustion air blower pressure switch. The control
will not begin the heating cycle if the pressure switch is
closed (by-passed). Once the pressure switch is deter
mined to be open, the combustion air blower is ener
gized. When the differential in the pressure switch is
great enough, the pressure switch closes and a 15-se
cond pre-purge begins. If the pressure switch is not
proven within 2-1/2 minutes, the control goes into
Watchguard-Pressure Switch mode for a 5-minute re-
set period.
After the 15-second pre-purge period, the SureLight
ignitor warms up for 20 seconds after which the gas
valve opens for a 4-second trial for ignition. The ignitor
stays energized for the first second of the 4-second
trial. If ignition is not proved during the 4-second peri
od, the control will try four more times. After a total of
five trials for ignition (including the initial trial), the con
trol goes into Watchguard-Flame Failure mode. After a
60-minute reset period, the control will begin the igni
tion sequence again.
The SureLight control board has an added feature that
prolongs the life of the ignitor. After a successful igni
tion, the SureLight control utilizes less power to ener
gize the ignitor on successive calls for heat. The control
continues to ramp down the voltage to the ignitor until
it finds the lowest amount of power that will provide a
successful ignition. This amount of power is used for
255 cycles. On the 256th call for heat, the control will
again ramp down until the lowest power is determined
and the cycle begins again.
TABLE 1
DIAGNOSTIC CODES
MAKE SURE TO ID LED'S CORRECTLY: REFER TO INSTALLATION INSTRUCTIONS FOR CONTROL BOARD LAYOUT.
LED #1 LED #2 DESCRIPTION
SIMULTANEOUS
SLOW FLASH
SIMULTANEOUS
SLOW FLASH
Power - Normal operation
Also signaled during cooling and continues fan.
SIMULTANEOUS FAST
FLASH
SIMULTANEOUS FAST
FLASH
Normal operation - signaled when heating demand initiated at thermostat.
SLOW FLASH ON
Primary or Secondary limit open. Units with board 63K8901 or 24L85: Limit must
close within 5 trials for ignition or board goes into one hour limit Watchguard. Units
with board 56L83: Limit must close within 3 minutes or board goes into one hour
limit Watchguard.
OFF SLOW FLASH
Pressure switch open or has opened 5 times during a single call for heat; OR:
Blocked inlet/exhaust vent; OR: Condensate line blocked; OR: Pressure switch
closed prior to activation of combustion air blower.
ALTERNATING SLOW
FLASH
ALTERNATING SLOW
FLASH
Watchguard - burners fail to ignite.
SLOW FLASH OFF
Flame sensed without gas valve energized.
ON SLOW FLASH
Rollout switch open. OR: 9 pin connector improperly attached.
ON
ON
OFF
ON
OFF
ON
Circuit board failure or control wired incorrectly.
FAST FLASH SLOW FLASH
Main power polarity reversed. Switch line and neutral.
SLOW FLASH FAST FLASH
Low flame signal. Measures below .7 microAmps. Replace flame sense rod.
ALTERNATING FAST
FLASH
ALTERNATING FAST
FLASH
Improper main ground or line voltage below 75 volts; OR: Broken ignitor; OR: Open
ignitor circuit.
NOTE - Slow flash equals 1 Hz (one flash per second). Fast flash equals 3 Hz (three flashes per second). Drop out flame sense current < 0.15 microAmps
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