Boiler shutting down on temperature: faulty sensor or genuinely too hot?

Short answer: if the boiler keeps shutting down with a temperature message, one of two things is going on: the boiler is genuinely getting too hot, or a sensor is measuring incorrectly. That distinction is everything. If it really gets too hot, there is too little flow and you need to solve that. If the sensor is measuring incorrectly, it is a part costing a few tens of euros. In either case, do not keep resetting.

What an NTC sensor does

A boiler has several temperature sensors: on the flow, on the return, often on the heat exchanger and, in a combi boiler, on the hot water. They are NTCs: resistors whose value changes with temperature. The controls convert that resistance into degrees. If such a sensor gets dirty, its connection corrodes or the wiring cracks, a nonsensical value comes in and the safety cut-out intervenes.

Typical: the boiler shuts down just after firing up, or it cuts out as soon as it reaches temperature. Sometimes it still works when cold but not when warm; that points to a sensor that only goes out of range at higher temperatures.

The codes per brand

Brand	Code	Message
Vaillant	F.05	Overheat fault, flow temperature too high
Vaillant	F.06	Flow NTC fault
Remeha	E05	Temperatuur warmtewisselaar te hoog door slechte doorstroming
Intergas	0 / 3	Sensor fault, hot water sensor S3
Bosch	A7	Hot water sensor open circuit or short-circuited

Note the difference in wording. "Sensor fault" or "open circuit" points to the part itself. "Temperature too high" or "overheat fault" means the reading is probably correct and the boiler really did get too hot; then the cause lies in the system.

If the boiler really gets too hot

A boiler that cannot get rid of its heat heats up within seconds. Causes: a circulation pump that is not running, air in the system, radiator valves that are all closed, or a fouled heat exchanger. You can recognise it because the flow temperature on the display rises extremely quickly and the boiler then shuts down immediately.

Find that cause first; a new sensor will not solve this. Our guide on radiators that stay cold goes through the flow step by step. Repeatedly resetting after overheating eventually damages the heat exchanger, and that is the most expensive part there is.

If the sensor is measuring incorrectly

Then it is usually a small and affordable part. An engineer measures the resistance of the sensor at a known temperature and compares it with the manufacturer's table; within a few minutes it is clear whether the sensor or the wiring is the problem. Often, by the way, it is not the sensor itself but the plug: corrosion or a connector that has half vibrated loose gives exactly the same message.

Replacement sensors can be found under temperature sensors and NTC sensors, pressure sensors under pressure sensors and the wiring under cables and wiring harnesses. Search using your boiler's rating plate, because sensors differ per series in length, clip shape and resistance value.

Why you should not keep resetting

A safety device that trips does so for a reason. With a temperature fault, the question is always whether the boiler got too hot or just wrongly thought so. In the first case, every reset makes things worse: you let the appliance heat up again without the heat being carried away. In the second case, a reset changes nothing, because the sensor keeps measuring incorrectly.

One reset to see whether it was a one-off is reasonable. If the message comes back after that, have it measured.

Frequently asked questions

How does an engineer know whether it is the sensor or the flow?

By measuring. They read the sensor value in service mode and compare it with a thermometer on the pipe. If they differ, it is the sensor; if they match and the temperature really is rising, it is the circulation.

Can I replace an NTC sensor myself?

They sit in the sealed part of the boiler, so it is work for a certified gas engineer. You can of course order the part yourself.

Why does my boiler only shut down in winter?

Because it then runs longer and harder; a sensor that only just stays within range at low load goes out of range at high load. The same applies to a system with too little flow: you only notice it when real output is demanded.

Mijn ketel geeft geen code maar valt wel uit. Wat nu?

Check whether the appliance is in a safety lockout or has simply lost power. Some brands only show a symbol. Note exactly what the display shows and look it up in the fault code database.