Air pressure switch not closing during pre-purge 1
The Air Switch Not Closed error code on Dunkirk boilers indicates air pressure switch not closing during pre-purge 1.
Dunkirk Air Switch Not Closed means: air pressure switch not closing during pre-purge 1. On this component the documented failure modes are no air transport to the burner after heat demand, flue or air inlet is blocked or aps is jammed or air signal hose not connected to the air intake pipe or wa- ter in hose or wiring, connector or harness of the pressure switch damaged or loose. You can safely reset the boiler once — if Air Switch Not Closed returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does Air Switch Not Closed Mean
The Air Switch Not Closed code is shown by the boiler’s control board when it detects air pressure switch not closing during pre-purge 1. 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 Air Switch Not Closed and the boiler stops until the fault is cleared |
Common Causes
The first cause below is the one the Dunkirk service literature publishes for this code (source: dunkirk.com). The others are the failure modes documented for the same component on other boilers, confirmed by 4 different manufacturers.
| Cause |
|---|
| no air transport to the burner after heat demand |
| flue or air inlet is blocked or APS is jammed or air signal hose not connected to the air intake pipe or wa- ter in hose |
| 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 |
Safety Warning
Air Switch Not Closed 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
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.
Have the heat exchanger inspected and cleaned; if it is blocked by limescale or sludge the system needs a chemical flush and an inhibitor dose.
Repressurise the system to the pressure stated on the boiler — typically 1.0–1.5 bar (15–22 psi) when cold. If it drops again, look for a leak on pipework, radiator valves and the pressure relief discharge.
Stop resetting: the underlying fault is still present. Switch the boiler off and on to restore the reset attempts, then have the original fault diagnosed.
Refer to the Dunkirk service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Dunkirk models that can display fault code Air Switch Not Closed:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Dunkirk models | — | Possible |
Frequently Asked Questions
Unplug and re-seat the connector of the pressure switch, then check the harness for chafing, corroded pins and breaks in continuity. According to the technical documentation, this is the first check on the list for Air Switch Not Closed.
Air Switch Not Closed is not classified as a high-severity fault, so the boiler can be reset once. If Air Switch Not Closed comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.