What This Error Means

The furnace control detects the second-stage pressure-switch circuit closed when it should be open. This code applies to the two-stage S9X2 furnace and is not applicable to S9X1 or S9B1 models.

Symptoms You May Notice

Second-stage heat may not start; furnace may remain in first-stage operation or shut down; the code can appear immediately when second-stage heat is requested; burners may not operate normally.

Common Causes

Second-stage pressure-switch contacts stuck closed; shorted or crossed wiring; water or debris affecting the switch or tubing; incorrect tubing connection; unintended draft or vacuum; integrated furnace-control input fault.

Safe Troubleshooting Steps

1. Turn off furnace power. 2. Confirm the model is an S9X2 before using this code. 3. Inspect second-stage pressure-switch wiring for pinched, crossed or shorted conductors. 4. Inspect the tubing for water, kinks, damage and correct routing. 5. Do not jumper the switch. 6. A qualified technician should verify the switch is open with no second-stage draft, measure the operating pressure and compare switch operation with the model-specific specification. 7. Test the control input if the switch and wiring operate correctly.

Parts Commonly Involved

Second-stage pressure switch; pressure tubing; inducer assembly; wiring harness; integrated furnace control.

When to Call a Technician

Yes. Diagnosis requires pressure-switch, draft and electrical testing on a two-stage furnace.

Safety Notice

Never bypass a pressure switch. It is part of the furnace’s combustion-venting safety system.

Manufacturer Documentation