Lennox Gas Furnaces: G51MP

Re: Pressure Switch Fault

Diagnostic Light Sequence:  DS1 (Red or Green) = Off, DS2 (Green) = Slow Flash

►There are two Differential-Pressure switches wired in series.

A Pressure Switch Fault is caused by one of the following:

• Either Pressure Switch contacts Open with Inducer Energized.
• Both Pressure Switch contacts Closed with Inducer Off.

CHECKOUT:

1. Check Furnace Operation

• If the inducer motor is running, de-energize a call for heat by turning the thermostat off, or by removing the control wire from "W" at the furnace board. (Inducer motor should stop after a delay).
  • Inducer Motor Continues to Run = Bad Control Board
  • Inducer Motor Stops ⇒ Continue
• Start the furnace with a call for heat.
• Inducer motor should start and run.
  • No Inducer + Pressure Switch Fault → Step 2. Check Inducer Motor
  • Inducer Runs + Pressure Switch Fault → Step 3. Check Pressure Switches

2. Check Inducer Motor

• With a call for heat, check for 115VAC between the two wires at the plug for the inducer motor.
  • Power + No Inducer = Bad Inducer / Capacitor or Blocked Inducer Wheel ⇒ Remove inducer and check wheel.
  • No Power = Pressure Switches Stuck Closed or Bad Control Board ⇒ Remove a wire from one of the pressure switches.
    • Inducer Starts = Pressure Switches Stuck Closed → Step 3. Check Pressure Switches
    • Still No Power = Bad Control Board

3. Check Pressure Switches

• Check for 24VAC between "24V Com" and Pin #1 (HLO) of the 12-pin plug at the furnace control board.
  • No Power = No Call for Heat or Bad Control Board
• Check for 24VAC between "24V Com" and Pin #10 (PS) of the 12-pin plug at the furnace control board.
  • Power on Pin #1 (HLO) + Power on Pin #10 (PS) + Pressure Switch Fault = Bad Control Board
  • Power on Pin #1 (HLO) + No Power Pin #10 (PS) = Open Pressure Switch Circuit ⇒ Check the Switch Contacts (below)
    • Switch Contacts
    • Check for 24VAC between "24V Com" and both sides of each Pressure Switch.
      • Power on Both Sides = Switch Closed
      • Power on One Side Only = Open Switch ⇒ Check Switch Pressure (below)
    • Switch Pressure
      • Unhook both pressure hoses from the open switch.
      • Use a pressure gauge to check the differential pressure with the inducer operating. (Trip Point = .85" w.c.)
        • Differential Pressure Well Above Trip Point + Open Switch = Bad Pressure Switch
        • Differential Pressure Near or Below Trip Point ⇒ Check the inducer and combustion box negative pressures individually.
          • High Combustion Box Pressure = Intake/Combustion Air Problem
          • Low Inducer Pressure = Vent/Vent Motor Problem or Clogged Drain
            • Remove the drain hoses from the plastic drain trap.
              • Water pours out of hoses = Clogged Drain or Trap
              • Check drain trap for a clog (clean by removing clean-out plugs)