Faulty hot water sensor: codes, testing with a multimeter and which sensor fits

Short answer: the cylinder sensor (also called a cylinder thermistor or cylinder NTC) measures the temperature in an indirectly heated hot water cylinder and tells the boiler or heat pump when the cylinder needs reheating. If it fails, the cylinder stays cold, or it gets too hot because the appliance keeps heating it, or you get a code such as Intergas 40, Vaillant F.03 or F.13, Nefit H12 or ATAG 203. The sensor costs € 19 to € 98, sits in a dry pocket with no water contact and is one of the few parts you can often replace yourself. Below are the codes, how to measure it and which sensor fits which appliance. If you have an electric water heater with its own element, read Electric water heater not producing hot water.

What the cylinder sensor does

An indirectly heated cylinder has no heating of its own; heating water from the boiler or heat pump flows through a coil in the tank. The sensor sits in a dry pocket in the cylinder wall, usually halfway up, and is an NTC: a resistor whose value drops as it gets warmer. At 10 kOhm the appliance reads, for example, 25 degrees; at about 1.2 kOhm, 60 degrees. If the measured temperature drops below the set cylinder temperature, the appliance switches the three-way valve to the cylinder and heats it. That is exactly why a sensor that measures incorrectly causes two opposite complaints.

The symptoms

What you notice	What the sensor is doing	What to do
Cylinder no longer heats up, heating works	Sensor reads too high or is short-circuited: the appliance thinks the cylinder is hot	Measure the resistance, replace the sensor
Cylinder gets scalding hot, appliance keeps heating it	Sensor reads too low or is open-circuit	Check sensor and cable; excessively hot tap water is a scalding risk, switch the cylinder function off temporarily
Code: cylinder sensor open or short-circuit	Broken wire, loose plug or faulty NTC	See codes below
Cylinder gets lukewarm, boiler switches back early	Sensor not pushed far enough into the pocket or measuring on the outside	Sensor helemaal inschuiven, zo nodig warmtegeleidende pasta
Cylinder hot, but tap water cold	Not a sensor problem	Mixing valve, inlet group or pipework; see no hot water

The codes per brand

Brand	Code	Message
Intergas	40 / 0-4	Sensor fault cylinder sensor S4
Nefit Bosch	H12	Cylinder temperature sensor faulty; boiler temporarily takes over the function
Vaillant	F.03	Cylinder temperature sensor signal interrupted
Vaillant	F.13	Cylinder temperature sensor short circuit
Bosch	1022 W	Cylinder temperature sensor faulty or contact problem
Bosch	Ad / A5	Cylinder NTC not recognised or faulty
ATAG	203	Cylinder sensor open circuit (system boiler with cylinder)
ATAG	bl 92 / bl 93	Cylinder sensor T6 open or closed (solar cylinder)
AWB	F.13	Cylinder sensor short circuit
Inventum (heat pump)	F-57	Cylinder temperature fault

Remeha boilers usually show a cylinder sensor fault as a general sensor code of the connected controller; look up the code on your display in the Remeha codes.

Measuring it yourself with a multimeter

Unplug the sensor (at the boiler or at the cylinder, wherever it is accessible) and measure the resistance across the two wires. Most cylinder sensors are 10 kOhm at 25 degrees and about 1.2 kOhm at 60 degrees; older Remeha Aqua cylinders use a 1 kOhm sensor. The exact table is in the installation manual of your appliance. If you measure infinity (open circuit) or zero (short circuit), the sensor or the cable is faulty. If you measure a normal value while the code persists, the problem is in the cable to the boiler or in the connection on the PCB.

Replacing it: you can do this yourself

The sensor sits in a sealed pocket; no water escapes. Unplug it, pull the old sensor out of the pocket, push the new one all the way in and route the cable back the same way. If the plug on the boiler side is behind the casing, have an engineer do that part: opening the casing of a gas appliance is a job for a certified gas engineer. On Nefit and Vaillant the connection is often on an external module or terminal strip that is accessible.

Which cylinder sensor fits which appliance?

The resistance value and the plug must match the appliance that controls the cylinder, not the cylinder itself. The parts below come from our catalogue; prices include VAT.

Brand	Part	Article number	Price
Intergas	NTC cold water/cylinder	8064010	€ 19.45
Remeha	Cylinder sensor Calenta Ace-Matic 40L, 10 kOhm	80103837	€ 13.95
Remeha	Cylinder sensor Aqua, Cella, 1 kOhm	8010018	€ 67.95
Nefit Bosch	Cylinder sensor with extension lead	0740034	€ 52.95
Nefit Bosch	Adapter cable external cylinder sensor	8027351	€ 29.95
Vaillant	Cylinder sensor uniSTOR VIH	8056639	€ 52.95
Bosch	Cylinder temperature sensor	8180040	€ 97.95
ATAG	Cylinder sensor E320S	0650037	€ 84.95
ATAG	NTC cylinder external SHR/Q series	8136372	€ 54.95
Inventum	Cylinder sensor	8771536	€ 25.95
Daikin	Cylinder sensor (heat pump)	91560171	€ 82.95

All sensors and NTCs are listed under temperature sensors. If you do not know which sensor you have, measure the resistance at room temperature and send us that value with a photo of the rating plate of the boiler or heat pump.

Frequently asked questions

Can I manage without a cylinder sensor for a while?

No. Without a valid sensor signal the appliance will not heat the cylinder, or with some brands will heat it without limit. Switch the cylinder function off until the sensor has been replaced; the heating will keep working as normal.

How hot should the water in the cylinder get?

For a home, set it to 60 degrees: hot enough to protect against legionella, not so hot that the mixing valve has to be fully open all the time. If the water gets much hotter without any change in settings, suspect the sensor.

Is the cylinder sensor in the cylinder or in the boiler?

In the cylinder, in a pocket in the wall. The cable runs to the boiler or heat pump and is connected there to the PCB or a terminal strip.

My cylinder is old – does the anode need replacing too?

Probably. The anode protects the tank against corrosion and is used up after five to eight years. Read Replacing a water heater anode; it is the cheapest part that will save your tank.