An E50 fault on a Baxi or Potterton boiler is quite an uncommon fault and isn’t readily found in the boiler manuals.
The E50 fault is caused by the hot water sensor going out of calibration.
The boilers control board expects to get a resistance reading from the hot water sensor that is within a range depending on how hot the water is.
When the sensor becomes faulty the resistance reading is outside the expected range which the control board doesn’t like.
The hot water sensor is screwed into the left hand manifold. It sits it the stream of the mains water after heating so it needs to be sealed using a washer.
To rectify this fault the sensor and seal should be replaced by a competent gas safe registered engineer.
With the water turned off and the boiler electrics isolated the gas safe engineer would pull off the green lead on the sensor.
Open a hot tap to release the water pressure then close it again to hold the water in the pipework.
Then using a small adjustable spanner unscrew the sensor from the pocket. There will be some water coming out but it shouldn’t be under pressure. There is a cut out under the sensor that a spanner fits through.
Fit the washer to the sensor and screw back into the boiler. Tighten it up but not too much as they can snap if tightened too much.
Reattach the lead, turn on the water and turn the power back on.