HVACopediaICP · Furnace
4+3 Low/medium pressure switch open while medium/high switch closed
What the control is telling you
Pressure switch states do not make sense: a lower switch is open while a higher one is closed.
Applies to: ICP F9MAE / G9MAE modulating variable-speed condensing gas furnaces (060, 080, 100, 120 kBtu/h)
Likely causes
- Low or medium pressure switch stuck open
- Disconnected or blocked pressure tubing
- Improper pressure switch wiring
- Low inlet gas pressure (if LGPS used)
- Plugged condensate drain
- Water in vent pipe (sagging)
Test and repair steps
- Check each pressure switch tube is connected and clear.
- Check pressure switch wiring against the wiring diagram.
- Flush collector box and clean condensate trap/drain; fix sagging vent pipe.
- If LGPS used, verify inlet gas pressure 4.5 to 13.6 in. w.c. (natural gas).
- Replace the stuck-open switch.
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Source: ICP (F/G)9MAE Service and Technical Support Manual 440 04 4700 00, Figure 16 Troubleshooting Chart, p.35. Summarized in our own words; check the original manual for your model.
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