Min air flow switch is open when it should be closed, or closed when it should be open
The MINIMUM AIR FLOW SWITCH FAULT error code on Weil-Mclain boilers indicates min air flow switch is open when it should be closed, or closed when it should be open.
Weil-Mclain MINIMUM AIR FLOW SWITCH FAULT means: min air flow switch is open when it should be closed, or closed when it should be open. On this component the documented failure modes are wiring, connector or harness of the flow switch damaged or loose, flue or air intake blocked or restricted or the flow switch has failed — short circuit or open circuit. You can safely reset the boiler once — if MINIMUM AIR FLOW SWITCH FAULT returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does MINIMUM AIR FLOW SWITCH FAULT Mean
The MINIMUM AIR FLOW SWITCH FAULT code is shown by the boiler’s control board when it detects min air flow switch is open when it should be closed, or closed when it should be open. 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 MINIMUM AIR FLOW SWITCH 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 flow switch damaged or loose |
| Flue or air intake blocked or restricted |
| The flow 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
MINIMUM AIR FLOW SWITCH 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 blower is operating correctly
Check to see if switch is not stuck open/closed
Check hose connection to switch for disconnect or damaged
Check wiring connections are firmly connected and not damaged
Check if Combustion Air Pressure Kit has been installed
Check sense line connections
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 MINIMUM AIR FLOW SWITCH FAULT:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Weil-Mclain models | — | Possible |
Frequently Asked Questions
Check blower is operating correctly. According to the technical documentation, this is the first check on the list for MINIMUM AIR FLOW SWITCH FAULT.
MINIMUM AIR FLOW SWITCH FAULT is not classified as a high-severity fault, so the boiler can be reset once. If MINIMUM AIR FLOW SWITCH FAULT comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.