The boiler has detected an implausible difference between the flow and return temperature sensors.
Most common on: ecoTEC plus, ecoTEC pro
F.72 means the control board is seeing a temperature reading it cannot make sense of from the flow and/or return sensors (the NTC sensors). Typically the difference between the two readings is larger than it should be, suggesting one sensor is giving a false value.
It usually points to a faulty flow or return temperature sensor, or a wiring fault, rather than a combustion problem. A recurring F.72 needs an engineer to test the sensors and their wiring.
Press reset. If F.72 clears and does not return, it may have been a transient glitch.
Sensor and wiring testing needs a heating engineer with a multimeter and the service manual — not a homeowner fix.
Because the fault involves flow and return temperatures, ask the engineer to check the pump and circulation as well, in case poor flow is skewing the readings.
Work on the gas side of a boiler must be carried out by a Gas Safe registered engineer. The checks above are limited to safe, non-invasive things a homeowner can look at (pressure, resets, the gas supply) — if in doubt, stop and call an engineer.
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The boiler has detected an implausible difference between the flow and return temperature sensors. F.72 means the control board is seeing a temperature reading it cannot make sense of from the flow and/or return sensors (the NTC sensors). Typically the difference between the two readings is larger than it should be, suggesting one sensor is giving a false value.
You can try a single reset and the safe checks below, but Vaillant F.72 usually needs a Gas Safe registered engineer. Never work on the gas side of a boiler yourself — it is illegal and dangerous unless you are Gas Safe registered.
A recurring F.72 means the underlying cause has not been fixed — most often faulty flow temperature sensor (ntc). Resetting repeatedly will not solve it; get the boiler diagnosed properly.