Lennox F8AUH series Service Manual page 26

PROBLEM 5: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER
Condition
5.1
- Unit operates with a cooling and con
tinuous fan demand.
- Combustion air blower energizes with Combustion air blower energizes with
Heating demand.
- Ignitor is energized but unit fails to
light.
LED#1-Alternating Slow Flash
LED#2-Alternating Slow Flash
PROBLEM 6: BURNERS LIGHT WITH A HEATING DEMAND BUT UNIT SHUTS DOWN
Condition
6.1
− Burners fire with a heating de-
mand.
− Burners light but unit shuts off prior
to satisfying T−stat demand.
− Diagnostic lights flash the pressure
switch code.
LED#1−Off
LED#2−Slow Flash
6.2
− Combustion air blower energizes
with a heating demand.
− Burners light but fail to stay lit. B li ht b t f il t
− After 5 tries the control diagnostics
flash the watchguard burners failed to
ignite code. ignite code.
LED#1−Alternating Slow Flash
LED#2−Alternating Slow Flash
ENERGIZES, IGNITOR IS ENERGIZED.
Possible Cause
Check that gas is being supplied to
Miswiring of gas valve or loose con-
nections at multi−pin control amp
Malfunctioning gas valve or ignition
PREMATURELY
Possible Cause
Low pressure differential at the pres-
Sensor or sense wire is improperly
t lit
Sensor or sense wire is broken.
Sensor or sensor wire is grounded to
Control does not sense flame.
5.1.1
the unit.
5.1.2
plugs or valve.
5.1.3
control.
6.1.1
sure switch.
6.2.1
installed.
6.2.2
6.2.3
the unit.
6.2.4
Page 26
Corrective Action/Comments
ACTION 1 − Check line pressure at the gas
valve. Pressure should not exceed 13" WC for
both natural and propane. Line pressure should
read a minimum 4.5" WC for natural and 8.0"WC
for propane.
ACTION 1 − Check for correct wiring and loose
connections. Correct wiring and/or replace any
loose connections.
ACTION 1 − Check that 24V is supplied to the
gas valve approximately 35 seconds after heat
demand is initiated.
ACTION 2 − Replace the valve if 24V is supplied
but valve does not open (check for excessive
gas line pressure before replacing gas valve).
ACTION 3 − Replace the control board if 24V is
not supplied to valve.
Corrective Action/Comments
ACTION 1 − Check for restricted vent inlet or ex-
haust. Remove all blockage.
ACTION 2: Check for proper vent sizing. See
installation instructions.
ACTION 1 − Check that sensor is properly lo-
cated (page 10) and that the sense wire is
properly attached to both the sensor and the
control.
ACTION 1 − Check for a broken sensor.
ACTION 2 − Test continuity across the sense
wire. If wire or sensor are damaged replace
the component.
ACTION 1 − Check for resistance between the
sensor rod and the unit ground.
ACTION 2 − Check for resistance between the
sensor wire and the unit ground.
ACTION 3 − Correct any shorts found in circuit.
ACTION 1 − Check the microamp signal from
the burner flame. If the microamp signal is be-
low 0.70 microamps, check the sense rod for
proper location or contamination.
ACTION 2 − Replace, clean, or relocate flame
sense rod. If rod is to be cleaned, use steel
wool or replace sensor. DO NOT CLEAN ROD
WITH SAND PAPER. SAND PAPER WILL
CONTRIBUTE TO THE CONTAMINATION
PROBLEM. NOTE: Do not attempt to bend
sense rod.