Fig. 72 – Troubleshooting Guide
1+2 BLOWERONAFTERPOWERUP–
(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.
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 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
opens within 5 minutes after ignition the
next heating cycle will be restricted to
maximum-heat. Check for:
- Proper vent sizing.
- Air leak between vestibule and blower
compartment.
- Lowinletgaspressure(ifLGPSused).
- Restricted vent.
- Disconnected or obstructed pressure
tubing.
- Defective or miswired pressure switches
- Excessive wind. If problem persists turn
setupswitchSW1-3ONtoboost
inducer speed
- Plugged condensate drain.
- Water in vent piping, possible sagging
pipe.
4 LIMIT CIRCUIT FAULT – Indicates the
limit or a flame rollout switch is open or the
furnace is operating in maximum-heat only
mode due to 2 successive limit trips while
operating at any rate <
71% of maximum
rate. 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.
- Improper minimum- or maximum-heat
gas input adjustment.
- Improper limit switch or no limit gasket.
- Dirty filter or restricted duct system.
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 thermostat connection from
the control, reset power, and put setup switch
SW1-6 in the ON position to start the
component test. Does the igniter glow
orange/white by the end of the 15 second warm-
up period?
Unplug igniter harness from control and
repeat component test by turning setup
switch SW1-6 OFF and then back ON.
Check for 115V between pin 3 and
NEUTRAL-L2 on the control. Was 115V
present for the 15 second period?
Check connections and retry.
If current is near typical value
(4.0-6.0 nominal) and burners
will not stay on, repeat check
in maximum-heat. If burners
will still not stay on replace
control. If burners operate in
maximum-heat then switch to
intermediate-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 microamps.
Is current near typical value?
Replace flame sensor.
Will main burners ignite and stay on?
Replace furnace control.
Fixed.
NO
YES
YES
YES
NO
NO
1 + 5 BLOWER MOTOR LOCKOUT –
Indicates the blower failed to reach 250
RPM or the blower failed to
communicate within 30 seconds after
being turned ON in two successive heat
cycles. Control will auto-reset after 3
hours. Refertostatuscode#4+1.
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 heat. Connect
voltmeter across gas valve BLUE and
GREEN/YELLOW wires. 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 unit to drop to min-heat,
and check manifold pressure.
Repeat test to check for
intermittent operation.
Check that all gas valves are
turned on. Replace valve.
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
period. Is the DC microamps below 0.5?
YES
YES
YES
YES
YES
NO
NO
NO
NO
NO
NO
YES
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.
- Gasvalveturnedoff.
- Manual shut-off valve.
- GREEN/YELLOW wire MUST be
connected to furnace sheet metal.
-Flamesensormustnotbegrounded.
2 + 5 INVALID MODEL SELECTION OR
SETUP ERROR – If status code 2 + 5
only flashes 4 times on power-up the
control is missing its model plug PL4
and is defaulting to the model selection
stored in memory. If status code 2 + 5
flashes continuously it could indicate
any of the following:
- Model plug PL4 is missing and there
is no valid model stored in permanent
memory. This will happen if you
forget to install the model plug PL4 on
a service replacement control.
- Thermostat call with SW1-1 ON.
- Thermostat call with SW1-6 ON.
- SW1-1 and SW1-6 both ON.
- Two different furnace models
twinned.
- Service replacement control is
incorrect. Need modulating board
with software version V17 or later.
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