F72 means your Vaillant no longer trusts its own temperature readings. The two sensors that measure the water going out and coming back disagree by too much, and Vaillant's manuals treat that as a sensor fault rather than a heating problem. It is a small part to replace, but it is inside the boiler, so it is not a DIY repair.
What F72 is telling you
Your boiler relies on two small temperature sensors, known as NTCs, clipped to the flow and return pipes. The control board uses them to decide how hard to fire the burner, when to stop and whether anything is overheating. Because both sensors sit on the same water circuit, their readings should always stay within a sensible range of each other.
When the gap between them grows larger than the circuit could realistically produce, the board concludes that one of the readings is wrong. It stops the boiler and shows F72, because firing on the strength of a false temperature reading could let the water overheat or cause the boiler to behave unpredictably.
Vaillant's ecoTEC plus installation and maintenance instructions (document 0020244997) describe the code as "Fault: Flow/return temperature sensor". The listed cause is that the temperature difference between the flow and return sensors is too great, with the conclusion that the flow sensor, the return sensor or both are defective.
Which models use F72, and how each manual words it
- ecoTEC plus VU and VUW ../5-5 (document 0020244997, covering the 825, 832, 835 and 838 combis and the matching system and regular boilers): "Fault: Flow/return temperature sensor".
- ecoTEC pro 24 and 28 combis (document 0020244995): "Fault: Flow/return sensor", with the cause given as the difference between the two sensors being too great.
- ecoTEC plus VU ../6-5 OVZ open-vent regular boilers (document 0020238195, 2021): the heading oddly reads "Deviation in the water pressure sensor/return temperature sensor", but the cause listed underneath is still the flow and return temperature difference.
- ecoTEC plus VU 246/6-5 OVZ LPG (document 0020239015): "Flow/return NTC fault".
If you have the 2021 open-vent manual and wonder why it mentions a water pressure sensor, go by the cause column. Every Vaillant table we checked links F72 to the temperature sensors.
F72 or F23: a faulty sensor or a flow problem?
Both codes are triggered by the gap between flow and return temperatures, which is why they are often mixed up. The difference lies in what Vaillant blames. F23 is a safety switch-off where the spread is real, caused by slow circulation from the pump or trapped air. F72 is a sensor fault, where the boiler suspects the reading itself.
There are also separate codes for a sensor that has failed outright. F00 covers an interruption in the flow sensor circuit, and F01 the return sensor. With F72, the sensor is still sending a signal, but one that does not make sense next to its partner.
F72 should not be confused with F75 either. F75 concerns the pressure sensor not seeing a pressure rise when the pump starts, so it points at the pump and water pressure side of the boiler, not at the temperature sensors.
What you can safely do
There is no homeowner fix for a defective sensor, but these steps help confirm the code and give the engineer a head start:
- Write down the code exactly as shown. If more than one code is active, the display cycles between them.
- Check the pressure gauge reads between 1 and 1.5 bar with the system cold. A normal reading helps rule out the pressure faults that can look similar.
- Reset the boiler once. On the 2021 and LPG ecoTEC plus models, Vaillant says to press and hold the reset button for more than three seconds.
- If F72 returns, try once or twice more at most, then leave the boiler and book an engineer.
Do not open the boiler to wiggle sensor plugs or clip a sensor back on. The sensors sit beside the burner, gas valve and mains wiring. A Gas Safe registered engineer should do this work.
How an engineer tracks down F72
The engineer's job is to find out which sensor is lying and why. A typical visit covers:
- Checking the sensor plugs and wiring for loose connections, corrosion or damage.
- Making sure each sensor is clipped firmly to its pipe, since a sensor that has slipped reads the air around it rather than the water.
- Comparing each sensor's reading with the real pipe temperature and with the values in Vaillant's service information.
- Replacing the sensor that is out of range.
As the two sensors were fitted at the same time, you can ask whether it makes sense to replace both during one visit. That is a judgement for the engineer, but asking can save a second call-out later.
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Refitting a sensor or repairing a plug or wire | Typically £100 to £180 |
| Replacing a flow or return temperature sensor (NTC) | Typically £100 to £200 |
| Extra labour, if the fault is hard to trace | Typically £50 to £80 an hour (London and the South East £90 to £110) |
A sensor fault is one of the cheaper repairs on a Vaillant, so F72 on its own is rarely a reason to replace a boiler.
Can you keep using the boiler with F72?
Usually you cannot, because the boiler locks out until the fault is cleared. That is deliberate. Do not try to get round it by repeatedly switching the power off and on. If you have a system or regular boiler with a hot water cylinder, an electric immersion heater, if fitted, can give you hot water until the repair. Combi owners will need to wait for the engineer, so mention when you book that the boiler is completely off.