The flue gas temperature sensor is giving a reading the boiler cannot use, so it has shut down.
Most common on: Baxi 600, Baxi 800, EcoBlue, Platinum+, Duo-tec
E28 means the flue gas thermistor (an NTC temperature sensor in the flue) is faulty or reading out of range. This sensor monitors the temperature of the gases leaving the boiler, and the control board will not run if it cannot trust that reading.
It usually points to the sensor itself, its wiring or connector, rather than a combustion problem. After a reset to rule out a glitch, a recurring E28 needs an engineer to test and typically replace the sensor.
Hold the reset button. If E28 clears and does not return, it may have been a transient glitch.
Make a note of whether the fault shows from cold or only after the boiler has been running — it helps the engineer diagnose the sensor.
Testing and replacing the flue gas thermistor and checking its wiring needs a Gas Safe engineer with a multimeter and the service manual — it is not a homeowner fix.
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 flue gas temperature sensor is giving a reading the boiler cannot use, so it has shut down. E28 means the flue gas thermistor (an NTC temperature sensor in the flue) is faulty or reading out of range. This sensor monitors the temperature of the gases leaving the boiler, and the control board will not run if it cannot trust that reading.
You can try a single reset and the safe checks below, but Baxi E28 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 E28 means the underlying cause has not been fixed — most often faulty flue gas ntc thermistor. Resetting repeatedly will not solve it; get the boiler diagnosed properly.
Flue gas thermostat / flue safety device
Overheat — primary circuit too hot
Fan fault
See all Baxi fault codes.