What This Error Means

The furnace control expected the second-stage pressure switch to close during second-stage operation, but the switch remained open. This code applies to S9X2 furnaces and is not applicable to S9X1 or S9B1 models.

Symptoms You May Notice

Furnace may heat only in first stage; second-stage burners may not start; inducer may change speed without completing the heating sequence; repeated attempts or a no-heat condition may occur.

Common Causes

Blocked intake or exhaust; restricted condensate drain or trap; water in pressure tubing; loose, cracked or kinked tubing; weak inducer performance at second stage; blocked pressure port; defective second-stage pressure switch; wiring fault.

Safe Troubleshooting Steps

1. Turn off furnace power. 2. Check the outdoor intake and exhaust openings for snow, ice, nests or debris. 3. Inspect the condensate trap and drain for blockage or poor routing. 4. Inspect second-stage pressure tubing for water, cracks, kinks and secure connections. 5. Confirm the inducer changes to the expected second-stage operation. 6. Do not bypass the pressure switch. 7. A qualified technician should measure draft pressure and verify the switch closes at the specified set point.

Parts Commonly Involved

Second-stage pressure switch; pressure tubing; inducer motor and wheel; intake and exhaust venting; condensate trap and drain; integrated furnace control; wiring.

When to Call a Technician

Yes, especially when the visible vent openings and condensate path are clear but the code returns.

Safety Notice

Never jumper or bypass the pressure switch. A persistent fault can indicate unsafe venting, condensate or inducer conditions.

Manufacturer Documentation