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Boiler Water pressure sensor: What It Does and Signs It's Failing

The water pressure sensor sits on the heating pipework inside the boiler casing. It tells the control board how much pressure is in the system so the boiler can warn you, and shut down, when it gets too low to run safely. Older boilers use a simple pressure switch that only knows on or off.

How it works

A sensor turns pressure into a small electrical signal that the board reads as a figure in bar. On a Vaillant ecoTEC pro the warning starts below 0.8 bar and the boiler stops at 0.5 bar. A pressure switch does the same job with a single contact that opens when pressure falls past its set point.

Signs it is failing

• Low-pressure code even though the gauge shows a healthy 1 to 1.5 bar
• Pressure figure on the display jumping around or stuck at one value
• Boiler refuses to start after you've topped up correctly
• Display reading that disagrees with a separate dial gauge

Low-pressure and pressure-rise faults name this sensor once real water loss has been ruled out. Vaillant also lists it among the causes of dry-fire and overheat lockouts.

Can you replace it yourself?

Your part is to read the gauge and top up through the filling loop as your user manual describes. If the code stays after the pressure is right, the sensor sits inside the sealed casing and needs a Gas Safe registered engineer in the UK or a Registered Gas Installer (RGI) in Ireland. Don't open the casing to look at it.

Buying the right part

Sensors and switches are specific to each boiler family, so the engineer needs the exact model and GC number from the data badge. A genuine manufacturer part is the safer choice here.

Error codes linked to the water pressure sensor

• E118Low water pressure in the heating circuitBaxi
• E119Low primary water pressure (Platinum Combi)Baxi
• E110 / E130Overheat lockout (primary water or flue)Baxi
• F.22Water pressure in the heating system is too lowVaillant
• F.20Overheat safety cut-out has trippedVaillant
• F.24Temperature rising too fastVaillant
• F.75No pressure rise when the pump startsVaillant
• F.83Dry fire: no temperature change when the burner litVaillant
• CE 207 / H07Heating system water pressure too lowWorcester Bosch
• E9 / E9 224Overheat limiter trippedWorcester Bosch
• A1Pump has run dryWorcester Bosch

Last reviewed 2026-10-21.