Low stage pressure switch stuck closed at start of heating

What is going on

The board sees the low stage pressure switch closed before the inducer has even started, so it won't go any further and the furnace won't run. The switch should be open when the inducer is off. If it reads closed at rest, it's stuck or something is shorting it.

Likely causes

• Low stage pressure switch circuit closed at the start of the heating cycle
• Switch contacts stuck closed
• Short in the switch wiring

Check in this order

1. Turn power off before repair.
2. Disconnect the switch leads and check across the switch with a meter. With the inducer off it should be open.
3. If it reads closed with the leads off, replace the pressure switch with the correct replacement part.
4. If the switch is open, check the wiring for a short.
5. Restore power and run a full cycle.

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