cfm Tech Tips – Misunderstood Flashes on Furnace Control Boards

Beginning January 1, 1992, all furnaces were manufactured with an integrated furnace control board, often referred to as FCB (Furnace Control Board). The FCB manages all operation requirements of the furnace. Fault and Information codes are displayed by one or more LEDs on the board. The FCBs used by York/JCI have the ability to store and retain the last 5 codes in ‘non-volatile memory’ – meaning that loss of power to the control doesn’t erase the stored codes – a very helpful tool when troubleshooting furnaces. This Tech Tips letter will begin a series of troubleshooting tips relating to Fault Codes on Furnace Control Boards.

We’ll begin with the Fault Code that is most misunderstood: 9 Red Flashes – Reversed Polarity.

Current Furnace Control Boards require correct polarity of the Line Voltage and the Low (control) Voltage applied to it.

  • With the introduction of FCBs in 1992, line voltage was not monitored by the FCB, but flame could not be proved with reversed polarity – thus the furnace would not work.
  • In 1998, line voltage monitoring was introduced along with Fault 9 – Reversed Polarity Notification. Line (Hot) must be landed on the L1 terminal and Neutral must be landed on the N terminal.
  • If they are reversed, the control can’t function correctly and will flash a Code 9 indicating Reversed Polarity.

Recently, FCB logic was enhanced to also monitor the polarity of the Low Voltage applied to the R and C terminals of the board. This is where Fault 9 troubleshooting can become more difficult. The polarity of the low voltage must be the same as the high voltage. If not, a Fault 9 will be flashed. This often occurs when an older FCB get replaced. The older control did not monitor low voltage polarity and the new one does. The voltages could be out of phase and the old control didn’t care, now the new control is looking at it and will not operate until the voltages are in phase.

If you just replaced an old Furnace Control Board and encounter a Fault 9, apply the following troubleshooting procedures to quickly resolve the problem. Please note that if the original FCB worked until it failed, the problem is probably not reversed line polarity – the original control would not have worked with reversed line polarity, but since we just replaced it – let’s begin with line voltage.

Troubleshooting steps to resolve Fault 9.

  1. With a volt meter make the following checks – this will confirm correct line polarity.
    • Line 1 to Neutral
      • Should be 120 volts (+/- 10%)
    • Line 1 to Case Ground
      • Should be 120 volts (+/- 10%)
    • Neutral to Case Ground
      • Should be less than 1 volt with the blower operating.
    • Note – Line Voltage Polarity can only be reversed at the incoming line connections.
  2. Now that Line Voltage Polarity is known to be correct, let’s check Low Voltage Polarity.

Important Notes

  1. When checking voltages in series, the measured voltage will be the sum of the series voltages.
  2. If they are in phase, the lower voltage will be subtracted from the higher voltage.
  3. If they are out of phase, the lower voltage will be added to the higher voltage.
  4. The following procedure will read high voltage in series with low voltage.
  1. With a volt meter make the following check of series voltages.
    • Place one meter lead on Line 1, place the other meter lead on the R (low voltage) terminal.
    • Reading should be Line Voltage minus Low Voltage (120V – 24V = 96 Volts approx.).
    • If Line 1 to R = 144 Volts (approx.) (120 + 24), the low voltage is out of phase and low voltage polarity must be reversed.
    • Low Voltage Polarity can only be reversed at the transformer terminals.
      • Apply Step 1 or Step 2 below – don’t do both or polarity will not be changed.
  2. Remove both high voltage wires from the transformer and swap them.
  3. Remove both low voltage wires from the transformer and swap them

Important Note – Do not swap a high voltage wire with a low voltage wire or you’ll let the smoke out of the control.

Coming next month: Pressure switch fault codes – Fault 2, Fault 3 and Fault 6.

Paul Flora
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Paul Flora

Service and Training Manager at cfm Distributors, Inc.
Paul has been with cfm Distributors, Inc. over 20 years and is currently the Service and Training Manager.His experience in the HVAC industry provides a high level of technical experience and knowledge for our customers.
Paul Flora
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