Air Pressure Switch Fault
The 166 error code on Baxi boilers indicates air pressure switch fault.
Baxi 166 means: air pressure switch fault. 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 166 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does 166 Mean
The 166 code is shown by the boiler’s control board when it detects air pressure switch fault. 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 166 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
166 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.
Step-by-Step Diagnosis
1. Check the boiler pressure gauge — normal operating pressure is 1.0-1.5 bar (15–22 psi) when cold. 2. Inspect all visible pipework, radiators, and valves for signs of leakage. 3. Check the pressure relief valve discharge pipe — if water is dripping, the PRV may be faulty or the expansion vessel has failed. 4. Test the expansion vessel — with the system cold and drained, check the Schrader valve on the vessel
air should escape at the correct pre-charge pressure (typically 0.75-1.0 bar, 11–15 psi). 5. Inspect the pressure sensor connections. 6. Check automatic air vents for signs of weeping
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 Baxi service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Baxi models that can display fault code 166:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Baxi 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 166.
166 is not classified as a high-severity fault, so the boiler can be reset once. If 166 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.