Lennox F8AUH series Service Manual page 24

PROBLEM 1: UNIT FAILS TO OPERATE IN THE COOLING, HEATING, OR CONTINUOUS FAN MODE
Condition
1.5
− On initial power−up the combustion
air blower remains energized.
− Diagnostic lights flash the improper
main ground. main ground
LED#1−Alternating Fast Flash
LED#2−Alternating Fast Flash
PROBLEM 2: UNIT FAILS TO OPERATE IN THE COOLING OR HEATING MODE, BUT COMBUS-
TION AIR BLOWER OPERATES CONTINUOUS. UNITS WITH CONTROL BOARDS DATE CODED
AFTER NOV.1 1997, WILL OPERATE IN COOLING WITH COMBUSTION AIR BLOWER CYCLING
5 SECONDS ON 55 SECONDS OFF, BUT NOT IN THE HEATING MODE.
Condition
2.1
− On initial power−up the combustion
air blower remains energized.
− Diagnostic lights flash the improper
main ground.
− Units with control boards date
coded after Nov.1 1997; combustion coded after Nov 1 1997; combustion
air blower will cycle 5 seconds on 55
seconds off.
LED#1−Alternating Fast Flash
LED#2−Alternating Fast Flash
PROBLEM 3: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER DOES
Condition
3.1
− Unit operates with a cooling or con-
tinuous fan demand.
− Combustion air blower will not start
with a Heating demand.
− Diagnostic lights flash the limit failure g g
mode. d
LED#1−Slow Flash,
LED#2−On
3.2
− Unit operates with a cooling and con-
tinuous fan demand.
− Combustion air blower will not start
with a Heating demand.
− Diagnostic lights flash the pressure − Diagnostic lights flash the pressure
switch failure code.
LED#1−Off,
LED#2−Slow Flash
Possible Cause
1.5.1
Improper ground to the unit.
1.5.2
6−Pin connector is improperly at-
tached to the circuit board.
Possible Cause
2.1.1
Open ignitor circuit.
2.1.2
Broken or failed ignitor.
NOT ENERGIZE
Possible Cause
3.1.1
Primary or secondary (if equipped)
limit open.
3.1.2
Miswiring of furnace or improper con-
nections at limit switch(es).
3.2.1
Miswiring of furnace or improper con-
nections to combustion air blower.
3.2.2
Pressure switch stuck closed.
Page 24
Corrective Action / Comments
ACTION 1 − Check that the unit is properly
grounded.
ACTION 2 − Install a proper main ground to the
unit
ACTION 1 − Check 6−pin connector for proper
installation. Correctly insert connector into con-
trol.
Corrective Action / Comments
ACTION 1 − Check for correct wiring and loose
connections in the ignitor circuit. Check multi −
plug connections for correct installation.
ACTION 1 − Unplug ignitor and read resistance
across ignitor. If resistance does not read be-
tween 10.9 and 19.7 ohms, replace the ignitor.
Corrective Action / Comments
ACTION 1 − Check continuity across switch(es).
Switches reset automatically upon cool down.
ACTION 2 − Check for restrictions on blower inlet
air (including filter) and outlet air. Determine
cause for limit activation before placing unit back
in operation.
ACTION 1 − Check for correct wiring and loose
connections. Correct wiring and/or replace any
loose connections.
ACTION 1 − Check for correct wiring and loose
connections. Correct wiring and/or replace any
loose connections.
ACTION 1 − Check that the pressure switch is
open without the combustion air blower operat-
ing. Replace if malfunctioning