62
Rev. 2
SYSTEM OPERATION
Abnormal Operation
(GUIV Variable Speed Two Stage Models with White-
Rodgers 50A51-255 Integrated Ignition Control)
The following presents the probable causes of questionable
furnace operation and how to fix them. Look through the
observation window in the blower access door and make a
note of the number of flashes in sequence between pauses.
Next, refer to the
DIAGNOSTIC SIGNAL CHART for an in-
terpretation of the signals and to this section for a descrip-
tion.
1. Internal Lockout. An internal lockout occurs when the
integrated ignition control senses an internal problem
and stops the unit.
To solve this problem, replace the ignition control.
2. External Lockout. A external lockout occurs when the
integrated ignition control determines that a measurable
combustion cannot be established or maintained after
three consecutive tries (four, if flame is established then
lost) to turn on the furnace.
If a flame is not sensed during the first seven seconds
after a gas valve has been energized, the ignition control
will internally turn off the gas. After 120 seconds, during
which time the induced draft blower purges the heat ex-
changer, the ignitor will reenergize and preheat for 27
seconds. The gas valve is then reenergized. If a flame is
not sensed again in seven seconds, the gas valve will
de-energize and another purge is performed. The igni-
tion control will cycle the gas valve
three times before it
determines it cannot establish measurable combustion
and enter a lockout state. If a flame is sensed but lost
after 10 seconds, the control will cycle
four more times
before locking out. A lockout stops ignition attempts and
causes the air blower to run continuously. The diagnos-
tic light code for either problem is one short flash fol-
lowed by a longer pause. The control can be reset and
brought out of lockout mode by turning the thermostat
off and then back on. It can also reset by turning off the
electrical disconnect switch to the furnace for 30 sec-
onds. The control will reset after two hours.
IMPORTANT: If you have to frequently reset your furnace, it
means that a problem exists that should be corrected. Con-
tact a qualified servicer for further information.
3. Pressure Switch Stuck Closed. A sticking pressure
switch can be caused by either a faulty pressure switch,
faulty wiring, a disconnected hose, or a restricted intake
or flue piping. In the case of a pressure switch sticking
closed, the probable cause is a faulty pressure switch
or wiring.
If the ignition control senses that the pressure switch is
closed and the induced draft blower is off, it will shut
down the unit until the fault is corrected. The light code
for this problem is two short flashes followed by a longer
pause.
4. Pressure Switch Stuck Open. A sticking open pres-
sure switch can be caused by a faulty pressure switch,
a disconnected hose to the pressure switch, a restricted
air intake or flue piping, or a faulty wiring.
If the ignition control senses that the induced draft blower
is energized, but the pressure switch is not closed, the
control will keep the blower on and wait for the switch to
close. The diagnostic light code for this problem is three
short flashes followed by a pause.
5. Open Thermal Protection Device. If the primary (high)
limit, auxiliary limit, or rollout limit control opens, the
gas valve is de-energized. Both the induced draft blower
motor and air circulating motor are energized on low heat
speed. The diagnostic light code for this problem is four
short flashes followed by a longer pause.
If the primary or auxiliary limit is open, the probable cause
is either: low conditioned air flow due to dirty filter or high
resistance in duct work, faulty limit, faulty blower, or blower
speed set too low. The primary limit will automatically
reset, while the auxiliary limit control must be manually
reset.
If the rollout limit control is open, the probable cause is
insufficient combustion air, restricted flue passage, or
restricted heat exchanger. The rollout limit control must
be manually reset.
6. Flame Sensed with No Call for Heat. If the control
senses a flame when the gas valve is de-energized it will
run the induced draft blower and air circulator blower con-
tinuously on the low heat speed for each. The diagnostic
light code for this problem is continuous flashing. The
probable cause is either a short to ground in flame sense
circuit or miswiring.
LIGHT SIGNAL
FOR CORRECTIVE ACTION REFER TO
ABNORMAL OPERATION NUMBER
Continuous Light
1. Internal Control Failure
1 Flash
2. System Lockout
2 Flashes
3. Pressure Switch Stuck Closed
3 Flashes
4. Pressure Switch Stuck Open
4 Flashes
5. Thermal Protection Device Open
Continuous Flashing
6. Flame Sensed No Call For Heat
Stat
Recovery
(1/4 Second on, 1/4 Second Off)
50A51 INTEGRATED IGNITION CONTROL
DIAGNOSTIC SIGNAL CHART
