High Efficient Furnace Troubleshooting

This terminal supplies 24 volts to the thermostat.

• R

• W

• Y

• G

• C

This terminal initiates the heating sequence of operation.

• R

• W

• Y

• G

• C

This terminal provides a path for voltage to ground.

• R

• W

• Y

• G

• C

A continuity test can be used to test the following:

• broken wires

• power to circuit

• grounded circuit

• wiring short

Refer to the Lennox MGF2 Wiring Diagram.

Assume the Thermostat is calling for heat and is providing power to Terminal W of the control board.

What voltage would you measure between

Terminal R and Terminal W of the Control Board?

Refer to the Lennox MGF2 Wiring Diagram.

Assume the Thermostat is calling for heat and is providing power to Terminal W of the control board.

What voltage would you measure between

Terminal C and Terminal W of the Control Board?

Refer to the Lennox MGF2 Wiring Diagram.

Assume the Thermostat is calling for heat and is providing power to Terminal W of the control board.

The Combustion blower is on...

What voltage would you measure at...

P20, Pin 9 & Pin 7?

Refer to the Lennox MGF2 Wiring Diagram.

Assume the Thermostat is calling for heat and is providing power to Terminal W of the control board.

What would happen if the wire broke off of J20, Pin 1?

Refer to the Lennox MGF2 Wiring Diagram.

There is no call for heat (W terminal de-energized).

What voltage would you measure across S18?

Reorder the following

• hsi

• flame sensor

• thermostat close

• pressure switch

• blower motor