Boiler water pressure sensor: sensor fault or genuinely low pressure, codes and which sensor fits

Short answer: if the boiler reports water pressure that is too low or too high while the pressure gauge shows a normal 1.5 to 2 bar, the water pressure sensor or its wiring is the suspect, not the system. Codes such as Remeha E.00.40, Nefit 1065, Vaillant F.73 and F.74 or AWB F 9 say exactly that: sensor open circuit, short circuit or out of range. A new sensor costs € 21 to € 54. If the pressure really is dropping, the problem is the expansion vessel or a leak, and you should read Water pressure keeps dropping. Below you can see how to tell the two apart and which sensor fits which appliance.

Sensor or switch?

Older boilers have a water pressure switch: a diaphragm with a contact that closes as soon as the pressure rises above about 0.5 to 0.8 bar. The boiler then only knows "enough pressure" or "too little pressure". Modern boilers have an electronic pressure sensor that measures the pressure continuously, usually 0 to 4 bar converted into a voltage of 0.5 to 3.5 volts. This lets the boiler show the pressure on the display, warn before it blocks and, with some brands, even top up automatically. Both are located on the hydro block or on the return pipe inside the boiler.

How to recognise a faulty sensor

What you see	Likely	What to do
Display shows 0.0 or an impossible value, pressure gauge reads 1.5 bar	Sensor or plug	Replace the sensor, check the wiring
Code "sensor open/short-circuit/out of range"	Sensor, cable or connector	See codes below
Code "pressure too low", display and gauge both low	Genuine pressure loss	Topping up, find the cause
Pressure high when hot and low when cold	Expansion vessel	Water pressure keeps dropping
Message comes and goes with vibrations or after the pump starts	Loose connector, break in the cable	Have the plug and cable checked
Pressure is fine but the boiler will not top up automatically	Sensor reads too high	Replace the sensor

A simple check you can do yourself: if the boiler has a display showing the pressure as well as a separate pressure gauge, compare the two. A difference of more than 0.3 bar points to the sensor. Top up a little and see whether the display follows; if the value stays the same, the sensor is not measuring.

The codes per brand

Brand	Sensor fault	Genuine pressure warning
Remeha	E.00.40 open/below range, E.00.41 short-circuit/above range, A.00.42 sensor not detected	A.02.06 pressure below minimum
Nefit Bosch	1065 sensor faulty or not connected	207 / 2E 207 pressure too low, 1017
Vaillant	F.73 open circuit, F.74 short circuit, F.09 sensor or wiring	F.21 too low, F.22 too high
Bosch	1065 W	CE207, 1017 W
AWB	F 9 sensor broken or wiring, F.73, F.74	21 too low, 22 too high
ATAG	102 sensor error, C 78 out of range or not connected, Bx12sc01 open, Bx12sc02 short-circuited	118 too low, 117 too high

Note the distinction the manufacturers themselves make. Remeha A.02.06 is a warning about the actual pressure, E.00.40 is about the sensor. With Nefit, 207 relates to the pressure and 1065 to the sensor. So do not order a sensor for every pressure message.

Why sensors fail

The sensor sits in the heating water. Sludge and magnetite settle in the small channel to the diaphragm, making it slow or inaccurate; that is why a dirty system produces sensor faults more often. Moisture also gets into the connector, especially on boilers where a siphon or a leaking coupling sits above the sensor. And with the small plug-in connectors, the contact can work loose through vibration. An engineer will therefore first check the plug and cable and measure the voltage before replacing the sensor.

Replacing it: what can you do yourself?

The sensor is a water-side part: no gas or flue gas is involved. Even so, on almost all boilers it is behind the casing, and opening and refitting the casing of a gas appliance is a job for a certified gas engineer. Our rule is therefore: have it done, but have the right part ready so it can be done in one visit. The work itself is quick: depressurise the boiler, release the retaining clip or union nut, remove the sensor, fit the new one with a new O-ring, refill, vent and check the reading. On Intergas the sensor is held with a clip and a retaining spring; order the spring at the same time.

Which sensor fits which appliance?

The sensors and switches below come from our catalogue; prices include VAT. Always check the appliance type; some brands have a screw-in and a clip-in version side by side.

Brand	Part	Article number	Price
Intergas	Heating pressure sensor, clip, service set	8063340	€ 31.95
Intergas	Service set pressure sensor, screw-in	8069577	€ 53.95
Intergas	Pressure sensor retaining spring	8063321	€ 6.95
Remeha	Pressure switch Avanta	8010259	€ 27.95
Remeha	Pressure sensor (Calenta, Tzerra)	8012610	€ 36.95
Remeha	Water pressure sensor	8010261	€ 48.95
Nefit Bosch	Pressure sensor	8029754	€ 33.95
Nefit Bosch	Pressure sensor ProLine NxT	8029513	€ 31.95
Nefit Bosch	Electronic pressure sensor with screw thread	8029080	€ 45.95
Vaillant	Pressure sensor VHR 18-22/3C, 24-28/3C	8058957	€ 20.95
Vaillant	Pressure sensor (ecoTEC)	80501434	€ 45.95
Bosch	Pressure sensor 6 bar	80200504	€38.95
AWB	Water pressure sensor	8077371	€35.95
AWB	Pressure sensor ThermoMaster	8070762	€ 32.95
ATAG	Pressure sensor water trap	8137098	€ 34.95
Itho Daalderop	CH water-side pressure sensor (also heat pump)	8170009	€ 37.95
Ferroli	Low water pressure protection Mega/Dens/Star	8113229	€38.95

All pressure sensors and switches are listed under pressure switches. If you cannot find your appliance, send us a photo of the rating plate and of the old sensor showing the printed number; we will look up the correct part.

Frequently asked questions

Can I use the boiler with a faulty pressure sensor?

Usually not: the boiler blocks because without a pressure reading it does not know whether there is water in it. Resetting helps only temporarily at best. If the pressure gauge reads correctly, there is no danger, but the boiler will stay off until the sensor has been replaced.

How does an engineer test the sensor?

With a multimeter: a supply voltage of 5 volts at the connector and an output voltage that follows the pressure, usually around 1.5 to 2 volts at 1.5 bar. No output voltage or a fixed value means a faulty sensor; no supply points to the cable or PCB.

Is a pressure sensor universal?

The measuring technology is the same for many brands (Huba Control), but the connection, the plug and the measuring ranges differ. So use the part for your appliance type; the wrong sensor gives an incorrect pressure reading and therefore new faults.

Why does my boiler only show a pressure sensor fault when starting up?

When the pump starts, the pressure changes briefly and the appliance vibrates. A connector that is only just making contact loses it at that moment. Have the plug checked and replace the cable if necessary; the sensor itself is often still fine.