Lennox Gas Furnace: G23Q4/5

Re: No Heat / Won't Ignite

This furnace uses a spark ignition control and electronic ignition (3-wire) gas valve.

• When the ignition control receives 24VAC on "TH" or "2" it should start sparking through the ignition cable and send power out "PV" or "1" to open the Pilot Valve of the gas valve.
• Once pilot flame is sensed, the ignition control should send power out "MV" or "3" to open the Main Valve of the gas valve and ignite the burners.

► The ignition control only senses pilot flame, so any ignition faults will be related to the pilot flame only.

CHECKOUT

1. Check Power

• Check for 24VAC between "R" and "C" on the low voltage wiring.
  • Power Present → Step 2. Check Ignition Sequence
  • No Power ⇒ Check the Blower Door Switch and High Voltage Power to Furnace
    • High Voltage + No Low Voltage ⇒ Check Transformer & Primary Limit (Open limit breaks power to "R").

2. Check Ignition Sequence

• Set thermostat to call for Heat
• Check 24VAC between "W" and "C" on the low voltage wiring.
  • No Power = No Heat Call ⇒ Re-Check Thermostat
  • Power + No Inducer = Bad Inducer or Bad Control Board
  • Inducer Running ⇒ Continue
• Check for 24VAC at the ignition control between "TH" or "2" and "GND" or "5".
  • Power Present → Step 4. Check Ignition Control
  • No Power or Power Drops-Out → Step 3. Check Safeties

3. Check Safeties

• With the thermostat calling for heat, check the control board and each of the following safeties:
• Control Board
• At the control board, check for 24VAC between "W1" and "C".
  • Power on "W" + No Power on "W1" = Bad Control Board
  • Power Present ⇒ Check Roll-Out Limit
• Roll-Out Limit
• Check for 24VAC between each side of the limit switch and "C".
  • Power on One Side Only = Limit Switch Open ⇒ Reset Limit & Check Heat Exchanger, Inducer Wheel and Vent
  • Power on Both Sides = Limit Switch Closed ⇒ Check Pressure Switch
• Pressure Switch
• Check for 24VAC between each side of the pressure switch and "C".
  • Power on Both Sides = Pressure Switch Closed
  • Power on One Side Only = Pressure Switch Open ⇒ Check Inducer Wheel & Vent Pressure

4. Check Ignition Control

• Once 24VAC is present on "TH" or "2" of the ignition control, the pilot should try to light (Unless in a Lockout).
  • Sparks but Does Not Light Pilot → Step 5. Check Gas Valve/Pilot
  • Pilot Lights but No Main Gas → Step 5. Check Gas Valve/Pilot
  • No Spark = Bad Ignition Control

5. Check Gas Valve/Pilot

• Check for 24VAC between "PV" and "GND" at the ignition control.
  • Power on "TH" + No Power on "PV" = Bad Ignition Control
  • Power Present ⇒ Continue
• Turn the gas valve to the "Off" position.
• Remove the pilot line from the gas valve and turn the valve back "On".
• Check for pilot gas leaving the gas valve.
  • Power on "PV" + No Pilot Gas = No Gas Supply or Bad Gas Valve
  • Pilot Gas Present ⇒ Re-attach pilot line, clean the pilot orifice and re-check.
• Make sure the pilot flame makes good contact with the flame sensor.
  • Little or No Contact ⇒ Increase Pilot Pressure
  • Good Contact + No Main Gas ⇒ Check for 24VAC between "MV" and "GND".
    • No Power = Bad Ignition Control
    • Power + No Ignition = No or Low Gas Supply or Bad Gas Valve ⇒ Check Gas Pressure