ICP F9MXT0401410A - Page 24

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
24 440 04 4321 04
Figure 16 (CONT.) Troubleshooting Guide − Flow Chart
1 + 2 BLOWER ON AFTER POWER UP –
(115V OR 24V) – Normal operation. Blower runs for 90 seconds, if unit is powered up during a call for heat (R- W/W1 closed) or when (R-
W/W1 opens)
during the blower on-delay period.
6 + 1 IGNITION LOCKOUT – System failed to
ignite gas and prove flame in 4 attempts. Control will auto-
reset after 3
hours. Refer to status code #6.
5 ABNORMAL FLAME-PROVING SIGNAL
Flame is proved while gas valve is de- energized. Inducer will run until fault is cleared. Check for:
- Stuck open or leaky gas valve.
2 PRESSURE SWITCH DID NOT OPEN –
Check for:
- Obstructed pressure tube.
- Pressure switch stuck closed.
3 LOW-HEAT PRESSURE SWITCH DID
NOT CLOSE OR REOPENED – If opens during blower on-delay period, blower will come on for the selected blower off-delay. If open longer than 5 minutes inducer shuts off for 15 minutes before retry. Check for:
- Proper vent sizing.
- Air leak between vestibule and blowe
r
compartment.
-
Low inlet gas pressure (if LGPS used).
- Disconnected or obstructed pressure
tubing.
-
Defective or miswired pressure switches
- Excessive wind. - Plugged condensate drain - Water in vent piping, possible sagging pipe - Restricted vent. - Defective Inducer motor. - Low inducer voltage (115 VAC)
4 LIMIT CIRCUIT FAULT – Indicates the
limit, flame rollout is open. Blower will run for 4 min. or until open switch remakes whichever is longer. If open longer than 3 min, code changes to lockout #7. If open less than 3 min. status code #4 continues to flash until blower shuts off. Flame rollout switch requires manual reset.
Check for:
- Loose blower wheel.
- Defective switch or connections. - Dirty filter or restricted duct system. - Improper gas input adjustment -
Improper limit switch or no limit gasket.
Unplug igniter harness from control and repeat component
test. Check for 115V
between
connector P2 (HSI) and
NEUTRAL-L2 on the control. Was 115V
present for the 15 second period?
Check connections and retry. If current
is near typical value
(4.0
-6.0μA DC nominal) and
burners will not stay on, repeat check in high
-heat. If
burners will still not stay on replace control. If burners operate in high
-heat then
switch to low-heat, check manifold pressure. If OK, check burner carryover and flame sensor location.
Clean flame sensor with fine steel wool and recheck current. Nominal current is 4.0 to 6.0
μA DC.
Is current near typical value?
Replace electrode.
Will main burners ignite and stay on?
Replace furnace control.
Fixed.
YES
YES
NO
NO
Replace furnace control.
Check for continuity in the harness and igniter. Replace defective component.
Reconnect the R thermostat lead and set thermostat to call for hea
t. Connect
voltmeter across gas valve connections. Does gas valve receive 24V?
Does gas valve open and allow gas to flow?
Do the main burners ignite?
Do the main burners stay on?
Allow blower to come on and repeat test to check for intermittent operation.
Check connections. If OK, replace control.
Check for: -
Inadequate flame carryover
or rough ignition.
-
Low inlet gas pressure.
-
Proper firing rate.
Repeat call for heat and check flame sensor current during trial for ignition peri
od. Is the DC current below 0.5μA
?
YES
YES
YES
YES
YES
NO
NO
NO
NO
NO
NO
6 IGNITION PROVING FAILURE – If flame is
not sensed during the trial for ignition period, the control will repeat the ignition sequence 3 more times before lockout #6 + 1 occurs. If flame signal is lost during the blower on-delay period, blower will come on for the selected blower off-delay. Check the following before going to the next step. - Gas valve turned off.
- Manual shut-off valve open?
-
Green/Yellow wire MUST be connected to furnace sheet metal.
- Flame sensor must not be grounded.
9 HIGH-HEAT PRESSURE SWITCH OR
RELAY DID NOT CLOSE OR REOPENED - Check for:
- Control relay may be defective. - Gas valve is miswired. - Refer to status code #3.
10 POLARITY – Check for correct line
voltage polarity. If units are twinned, check for proper low-voltage (24V) transformer phasing.
7 LIMIT CIRCUIT LOCKOUT – Lockout
occurs if the limit or flame rollout is open longer than 3 minutes. Control will auto- reset after 3 hours. Refer to status code #4.
8 GAS HEATING LOCKOUT – Control will
NOT auto reset. Turn off power and wait
5 minutes to retry. Check for: -
Stuck closed gas valve relay on control.
- Miswire or short to gas valve wire.
YES
Check that gas valve electric switch is turned
on. Replace
valv
e.
To determine whether the problem is in the gas valve, igniter, or flame sensor the system can be operated in component test mode. To check the igniter, remove the R ther
mostat connection from the
control, reset power, start the component test. Does the igniter glow orange/white by the end of the 15 second warm
-up
period?
NO
YES