MG3P Installation Manual

33
DIAGNOSTIC DESCRIPTION
GREEN
LED
RED
LED
Control Fault (No Power) Off Off
L1/Neutral Polarity Fault Flash Flash
1 Hour Lockout Alternating Flash
Normal Operation On On
Pressure Switch Closed Fault On Flash
Pressure Switch Open Fault Flash On
Open Limit Switch Fault Flash Off
DIAGNOSTIC DESCRIPTION YELLOW LED
Low Flame Sensor Signal Continuous Flash
Flame Present On
Table 6. MG3P Control Board Fault Codes
MG3S OPERATING SEQUENCE
The operating sequences for the heating, cooling, and fan modes
are described below. Refer to the field and furnace wiring and
ladder diagrams: Figure 45 (page 42), Figure 46 (page 43),
Figure 47 (page 44), & Figure 48 (page 45)
Heating Cycle
1. The thermostat calls for heat by energizing the W1 terminal
with 24VAC.
2. The control checks to see the pressure switch is open. If the
switch is closed, the furnace will shut down for 5 minutes
before retrying
3. If the pressure switch is open, the control energizes the
inducer motor and waits for the pressure switch to close. The
pressure switch must close within 12 seconds.
4. The control runs the inducer for a 30 second pre-purge time.
5. The control energizes the igniter output for the appropriate
adaptive warm-up time limit.
6. The furnace always ignites the burners in high fire. If the call
for heat is for low rate, the furnace will move down to low fire
after the flames stabilize.
7. If the flame is proved and ignites the gas, the control de-
energizes the igniter. The gas valve and inducer remains
energized. The control goes to blower on delay.
8. The control energizes the blower on the selected HEAT speed
22 seconds after the gas valve opened. The gas valve and
inducer remain energized.
9. If there is a call for high fire, the gas valve moves to the high
fire position and the blower speeds are increased. The furnace
will remain in high fire until the demand for heat is satisfied.
10. If autostaging is enabled (single stage thermostat) the demand
for heat has lasted more than the selected time, the furnace
automatically moves up to high fire. Autostage time is ON
(10 minutes) or OFF, depending on the Jumper (P7) setting
on the furnace control board.
11. When the thermostat demand for heat is satisfied, the control
de-energizes the gas valve. The inducer output remains on
for a 30 second post-purge period.
12. The circulating air blower will continue to run for the selected
Blower Off Delay(P5). This may be 60, 90, or 120 seconds
depending on the jumper setting on the furnace control board.
Cooling Cycle
1. The thermostat calls for cooling by energizing the Y / Y2 or
Y1 terminal with 24VAC.
2. The control energizes the blower in the cooling speed and
sends 24VAC to the contactor in the condensing unit.
3. When the thermostat removes the call for cooling, the contactor
in the outdoor condensing unit is de-energized and the control
continues to run the fan for a period of 60 seconds.
Fan Mode
• When the thermostat energizes the G terminal for continuous
fan (without calling for heat or cooling), the indoor fan is
energized on the selected FAN speed.
• If a call for cooling occurs during continuous fan, the blower
will switch over to the selected COOL speed.
• If the W terminal receives a call for heat during continuous
fan, the blower will de energize.
• A call for fan is ignored while in lockout.
• For continuous fan operation, the blower motor will operate at
35% of the selected high-speed torque when equipped with
a fixed speed motor control board.
TROUBLESHOOTING
If the furnace fails to operate check the following:
• Is the thermostat operating properly?
• Are the blower compartment door(s) in place?
• Is the furnace disconnect closed?
• Has the circuit breaker tripped or the control board fuse burned
open?
• Is the gas turned on?
• Are any manual reset switches open?
• Is the filter dirty or plugged?
• Is the flame sensor coated? Remove and clean with steel
wool. (Do not use emery cloth or sandpaper!)
• Is there blockage in the condensate drain switch? Also verify
that there is no double trapping of condensate.
• Is the secondary heat exchanger free of debris and clogs?
• Is evaporator coil clean and free of debris (If applicable).
• Are all the LED’s on the furnace control board constantly
ON? If not, refer to Table 6 (page 33) or the wiring diagram
(Figure 45 (page 42)) to determine fault condition.
IMPORTANT NOTE:
The furnace will lock out after 5 failed attempts for ignition
and will try again every hour if the call for heat remains.
• If the Inducer Blower is operating, and items above have been
verified, check the blower limit switch and reset if necessary.
See Figure 55 (page 54) for component location.
• If the furnace operates when the blower limit switch is reset,
contact a qualified service technician to identify and repair
the problem.
• If the furnace still doesn’t operate, check the flame roll-out
switches (Figure 55) and reset if necessary.
• If the furnace operates when the flame rollout switch is reset,
contact a qualified service technician to identify and repair
the problem.