Gas pressure switch is open (high or low)
The GAS PRESSURE SW FAULT error code on Weil-Mclain boilers indicates gas pressure switch is open (high or low).
Weil-Mclain GAS PRESSURE SW FAULT means: gas pressure switch is open (high or low). On this component the documented failure modes are wiring, connector or harness of the pressure switch damaged or loose, flue or air intake blocked or restricted or the pressure switch has failed — short circuit or open circuit. You can safely reset the boiler once — if GAS PRESSURE SW FAULT returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does GAS PRESSURE SW FAULT Mean
The GAS PRESSURE SW FAULT code is shown by the boiler’s control board when it detects gas pressure switch is open (high or low). The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows GAS PRESSURE SW FAULT and the boiler stops until the fault is cleared |
Common Causes
The manufacturer does not publish a cause list for this specific code. The causes below are the failure modes documented for the same component on other boilers, confirmed by the service literature of 4 different manufacturers.
| Cause |
|---|
| Wiring, connector or harness of the pressure switch damaged or loose |
| Flue or air intake blocked or restricted |
| The pressure switch has failed — short circuit or open circuit |
| Heat exchanger fouled or partially blocked |
| System water pressure too low, or a leak somewhere in the system |
| Too many reset attempts: the boiler has locked itself out |
Safety Warning
GAS PRESSURE SW FAULT involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
How to Fix It
Check gas pressure at tee mounted behind gas pressure switches during high and low fire and boiler off. Pressure should always be with limits on rating label. Undersized gas supply pipes or regulators may cause low gas pressure issues while the boiler is operating and possibly high gas pressure when the boiler shuts down
Insure the pressure regulator in the gas supply line is a lockup-type
Ensure gas pressure switch settings are correct. Low Pressure Switch Setting = 2 inches H2O High Pressure Switch Setting = 14 inches H2O
Unplug and re-seat the connector of the pressure switch, then check the harness for chafing, corroded pins and breaks in continuity.
Inspect the flue and air intake along their whole length, including the outside terminal, and clear any obstruction or debris.
Measure the resistance of the pressure switch with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range.
Refer to the Weil-Mclain service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Weil-Mclain models that can display fault code GAS PRESSURE SW FAULT:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Weil-Mclain models | — | Possible |
Frequently Asked Questions
Check gas pressure at tee mounted behind gas pressure switches during high and low fire and boiler off. Pressure should always be with limits on rating label. Undersized gas supply pipes or regulators may cause low gas pressure issues while the boiler is operating and possibly high gas pressure when the boiler shuts down. According to the technical documentation, this is the first check on the list for GAS PRESSURE SW FAULT.
GAS PRESSURE SW FAULT is not classified as a high-severity fault, so the boiler can be reset once. If GAS PRESSURE SW FAULT comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.