Carrier 90% Gas Furnace: 350MAV

Re: Flashing Code 34 / Fault Code 34

Code 34 = Ignition Faulure

Diagnostic Light Sequence: 3-Short Flashes, 4-Long Flashes (- - - , — — — —)

► 4 Consecutive Ignition Faults will become a Fault Code 14.

CHECKOUT:

1. Check Ignition Sequence

• If the furnace is running, turn the thermostat off and allow the furnace to cool and the blower to stop operating.
• Initiate a call for heat from the thermostat and monitor the ignition sequence.
  • 1. The inducer motor should start with no difficulty.
  • 2. The pressure switch should close within a couple seconds of the inducer starting.
  • 3. The glow plug should energize within 30 seconds of the pressure switch closing.
    • No Glow from Glow Plug → Step 3. Check Glow Plug
  • 4. The gas valve should open within 20 seconds of the glow plug energizing.

2. Check Ignition

• Visually inspect the burners during ignition.
  • The burners should light smoothly with little or no delay from the glow plug to the flame sensor.
  • Make sure the flame sensor comes in good contact with the flame.
    • Burners Ignite Properly → Step 5.  Check Flame Sensor
    • Any Delay or Burners Not Igniting = Clogged Burners, Low Gas Pressure or Bad Gas Valve → Step 4.  Check Gas Pressure / Valve

3. Check Glow Plug

• Look for any visual cracks.
• Unplug the 2-wire connector to the glow plug.
• Use a meter to test resistance of the glow plug.
  • No Resistance = Bad Glow Plug
• Start an ignition sequence.
• Check for 120VAC power on the 2-wire plug feeding the glow plug during an ignition sequence.
  • No Power = Bad Ignition Control
  • Power Present + No Glow = Bad Glow Plug

4. Check Gas Pressure / Valve

• Inlet Gas Pressure
• Check the inlet gas pressure with the furnace off.
• Minimum Pressure = 5" w.c. for Nat. gas, and 11" w.c. for propane.
  • Low Pressure = Bad Meter or Regulator
• Monitor the inlet gas pressure as the furnace attempts to ignite.
  • Pressure Drops Below Minimum = Bad Meter/Regulator or Restricted Gas Line
• Manifold Gas Pressure
• Check the manifold pressure when the furnace attempts to ignite.
  • Normal Pressure = 3 - 3.5" w.c. for Nat. Gas, 9 - 11" w.c. for LP
    • No Gas Pressure = Bad Gas Valve
    • Low Gas Pressure ⇒ Increase Manifold Gas Pressure
    • Normal Gas Pressure + No or Delayed Ignition:
      • Combustion Air Issue ⇒ Re-try an ignition with the burner cover removed.
      • Clogged Burners ⇒ Remove burners & clean spreaders (especially in front of glow plug).

5. Check Flame Sensor

• On the multi-meter:
  • Switch the test leads to "COM" and "mA".
  • Set the meter on DC 200ù.
  • Disconnect the wire from the flame sensor.
  • Place one meter lead on the flame sensor terminal and the other on the flame sensor wire.
  • Check the reading when the burners ignite.
    • Normal Reading = 4 to 6 mA
    • Lockout Point = 0.5 MicroAmp
      • Low Reading = Dirty Flame Sensor ⇒ Clean Flame Sensor & Re-check
  • Continue to monitor the flame and flame signal through a burner cycle.
    • Individual Burners Extinguish or Flutter = Combustion Air Issue or Bad Heat Exchanger ⇒ Re-try a burner sequence with the burner cover removed.
  • CAUTION! Move meter leads back to "COM" and "V" before proceeding!