Lennox International Inc. MPB35ST-NM - Manual (Page 12)

NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
12
TROUBLESHOOTING GUIDE - MILLIVOLT GAS CONTROL SYSTEM
Note: Before troubleshooting the gas control system, Ensure external gas shut off valve, located at gas supply inlet, (and wall switch, iff applicable), is
in the “ON” position. Important: Valve system troubleshooting should only be accomplished by a qualifi ed service technician.
SYMPTOM POSSIBLE CAUSES CORRECTIVE ACTION
1. Spark igniter will not light pilot after
repeated triggering of igniter button.
A. Defective igniter
(no spark at electrode).
Check for spark at electrode and pilot; if no spark and
electrode wire is properly connected, replace igniter.
B. Defective or misaligned electrode at pilot
(spark at electrode).
Using a match, light pilot. If pilot lights, turn off pilot
and trigger the igniter button again. If pilot lights, an
improper gas mixture caused the bad lighting and
a longer purge period is recommended. If pilot will
not light – check gap at electrode and pilot – should
be 1/8" to have a strong spark. If gap measures 1/8",
replace pilot (see Figure 17 on Page 14).
C. Gas supply pressure errant. Check inlet gas pressure. It should be within the limits
as marked on the rating plate.
D. Pilot orifi ce plugged. Clean or replace pilot orifi ce.
2. Pilot will not stay lit after
carefully following the lighting
instructions.
A. Defective pilot generator
(thermocouple). Check pilot fl ame, it must impinge on thermocouple
(see Figure 17 on Page 14). Clean and/or adjust pilot
for maximum fl ame impingement on thermocouple.
Ensure that the connection between the valve and
thermocouple are tight and secure.
3. Pilot burning, no gas to burner, Valve
knob “ON,” and the (standard) burner
OFF/ON switch is “ON." Read impor-
tant note below.
IMPORTANT NOTE: If an optional
*remote switch is used for burner
operation, the standard burner
OFF/ON switch on appliance must
be in the "OFF" position.
*Optional remote switch kits - wall
switch or remote control.
A. Manual Reset Spill Switch has Tripped or is
defective.
Follow Manual Reset procedure on Page 6. Once the
red reset button is fully depressed and clicks in, check
continuity with a multimeter between switch terminal
(Ohms). If it shows an open circuit, replace switch.
B. Wall switch or wires defective. Check wall switch and wires for proper connections.
Jumper wire across terminals at wall switch, if burner
comes on, replace defective wall switch. If okay, jumper
wires across wall switch wires at valve, if burner comes
on, wires are faulty or connections are bad.
C. Thermopile may not be generating suf-
fi cient millivolts.
Check thermopile with millivolt meter. Take reading
at thermopile terminals of gas valve. Should read
325 millivolts minimum with optional wall switch
“OFF.” Replace faulty thermopile if reading is below
specifi ed minimum.
D. Plugged burner orifi ce. Check burner orifi ce for blockage and remove.
E. OFF/ON Switch & *Remote Switch are in
the "ON" position resulting in excessive
resistance.
When turning on the burner using a *remote switch,
ensure that the standard OFF/ON switch is in the "OFF"
position. If both switches are in the ON position, it
may result in excessive resistance (& millivolt drain-
age) and the burner may not come on.
4. Frequent pilot/burner outage prob-
lem.
A. Pilot fl ame may be too low or blowing (high)
causing the pilot/valve safety to drop out.
Clean and/or adjust pilot fl ame for maximum fl ame
impingement on thermocouple (see Figure 19 on
Page 20).
TROUBLESHOOTING
SYMPTOM POSSIBLE CAUSES CORRECTIVE ACTION
1. Burner will not light. A. Faulty Valve System. See Below.
B. “OFF/ON” or wall switch defective. Disconnect the two black wires from the wire nuts.
Test switch(s) for continuity with a multimeter. If
continuity is not indicated, switch(s) is defective and
must be replaced.
Note: Before replacing “OFF/ON” switch, Ensure to
check wiring for loose connections or broken wires
and repair as needed.
2. Burners come “ON,” then turns “OFF.” A. Burner orifi ce plugged. Check main burner orifi ce(s) for blockage. Clean
or replace.
B. Obstructed vent system. Check vent system for obstructions.
TROUBLESHOOTING GUIDE - ELECTRONIC IGNITION SYSTEM