Gas valve (two-stage) and front cover pressure switch

What is going on

The two-stage valve needs 24V between common and the low fire (main) coil before it will open. The front cover pressure switch sits in series with that low coil, so if that switch is open the valve gets no voltage and acts dead. Many valves get replaced when the real problem is the switch circuit.

Likely causes

• Front cover pressure switch open, from a blocked or split hose, or a tripped switch
• No 24V between common and low coil, from wiring or control problem
• Non-functioning gas valve with 24V present at the coil

Check in this order

1. Start with the front cover pressure switch if the valve does not open, because it is in series with the coil
2. Check the hose to the switch for water, cracks or blockage
3. Confirm 24V between common and low (main) coil when a heat call is present, because that tells you if the valve is getting what it needs
4. If 24V is present and the valve stays closed, the valve is the problem
5. If 24V is absent, trace backwards through the switch and the control
6. Refer to the gas pressure and pressure switch checks for further testing
7. Gas valve replacement is licensed gas tech work, so leave it to them

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