Lennox International Inc MPB35ST-NM Troubleshooting, Symptom, Possible Causes, Corrective Action

Models: MPB35ST-NM

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TROUBLESHOOTING

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TROUBLESHOOTING

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 qualified service technician.

TROUBLESHOOTING GUIDE - MILLIVOLT GAS CONTROL SYSTEM

 

SYMPTOM

POSSIBLE CAUSES

CORRECTIVE ACTION

1.

Spark igniter will not light pilot after

A. Defective igniter

Check for spark at electrode and pilot; if no spark and

 

repeated triggering of igniter button.

(no spark at electrode).

electrode wire is properly connected, replace igniter.

 

 

B. Defective or misaligned electrode at pilot

Using a match, light pilot. If pilot lights, turn off pilot

 

 

(spark at electrode).

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 orifice plugged.

Clean or replace pilot orifice.

2.

Pilot will not stay lit after

A. Defective pilot generator

Check pilot flame, it must impinge on thermocouple

 

carefully following the lighting

(thermocouple).

(see Figure 17 on Page 14). Clean and/or adjust pilot

 

instructions.

 

for maximum flame impingement on thermocouple.

 

 

 

Ensure that the connection between the valve and

 

 

 

thermocouple are tight and secure.

3.

Pilot burning, no gas to burner, Valve

A. Manual Reset Spill Switch has Tripped or is

Follow Manual Reset procedure on Page 6. Once the

 

knob “ON,” and the (standard) burner

defective.

red reset button is fully depressed and clicks in, check

 

OFF/ON switch is “ON." Read impor-

 

continuity with a multimeter between switch terminal

 

tant note below.

 

 

 

(Ohms). If it shows an open circuit, replace switch.

 

 

 

 

 

 

 

 

IMPORTANT NOTE: If an optional

B. Wall switch or wires defective.

Check wall switch and wires for proper connections.

 

 

Jumper wire across terminals at wall switch, if burner

 

*remote switch is used for burner

 

comes on, replace defective wall switch. If okay, jumper

 

operation, the standard burner

 

wires across wall switch wires at valve, if burner comes

 

OFF/ON switch on appliance must

 

on, wires are faulty or connections are bad.

 

be in the "OFF" position.

 

 

 

C. Thermopile may not be generating suf-

Check thermopile with millivolt meter. Take reading

 

 

 

*Optional remote switch kits - wall

ficient millivolts.

at thermopile terminals of gas valve. Should read

 

switch or remote control.

 

325 millivolts minimum with optional wall switch

 

 

 

“OFF.” Replace faulty thermopile if reading is below

 

 

 

specified minimum.

 

 

D. Plugged burner orifice.

Check burner orifice for blockage and remove.

 

 

 

 

 

 

E. OFF/ON Switch & *Remote Switch are in

When turning on the burner using a *remote switch,

 

 

the "ON" position resulting in excessive

ensure that the standard OFF/ON switch is in the "OFF"

 

 

resistance.

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-

A. Pilot flame may be too low or blowing (high)

Clean and/or adjust pilot flame for maximum flame

 

lem.

causing the pilot/valve safety to drop out.

impingement on thermocouple (see Figure 19 on

 

 

 

Page 20).

TROUBLESHOOTING GUIDE - ELECTRONIC IGNITION SYSTEM

 

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 orifice plugged.

Check main burner orifice(s) for blockage. Clean

 

 

 

 

or replace.

 

 

 

 

 

 

B. Obstructed vent system.

Check vent system for obstructions.

 

 

 

NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.

 

 

 

 

 

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Lennox International Inc MPB35ST-NM Troubleshooting, Symptom, Possible Causes, Corrective Action, be in the OFF position