3 Pressure switch did not close or reopened

What the control is telling you

9MXE: the collector-box (LPS) or inducer-housing (HPS) switch did not prove or opened again. 9MXT: the low-heat switch did not prove or opened again. If it opens during the blower-on delay the blower runs through its off delay. If open over 5 minutes the inducer stops for 15 minutes and retries. 9MXE: if the HPS stays open 1 minute, the furnace locks out for 3 hours after repeated tries.

Applies to: KeepRite (F/G)9MXE single-stage and (F/G)9MXT two-stage 95% furnaces

Likely causes

• Vent pipe undersized or restricted, or water lying in a sagging vent
• Plugged condensate drain
• Air leak between the vestibule and blower compartment
• Disconnected or blocked pressure tubing
• Bad or miswired pressure switch
• High wind at the terminal
• Weak inducer or low 115 V at the inducer
• Low inlet gas pressure (only where a low gas pressure switch is fitted)

Test and repair steps

1. Check the vent and intake for correct size, blockage and water pockets; regrade any sagging pipe.
2. Clear the condensate trap and drain.
3. Inspect pressure tubing and reconnect or clear it.
4. Confirm the inducer comes up to speed and has 115 V.
5. Seal any leak between vestibule and blower compartment.
6. Check switch wiring, then test the switch with a manometer before replacing.
7. If a low gas pressure switch is used, measure inlet gas pressure.

Settings for these units

• Filter size by casing width · KeepRite (F/G)96VTN Two-Stage Variable-Speed Furnace

Models that show this code

16 models in KeepRite (F/G)9MXE Single-Stage Furnace, KeepRite (F/G)9MXT Two-Stage Furnace

Other codes on the same units

• 1+2 Blower running after power-up (normal)
• 2 Pressure switch stuck closed (did not open)
• 4 Limit or rollout circuit open
• 5 Flame sensed with gas valve off
• 6 Flame not proved during ignition trial
• 6+1 Ignition lockout after 4 failed tries
• 7 Limit circuit lockout
• 8 Gas heating lockout - manual reset needed