Hi Ivan,
Try degaussing the Hall sensor on the rpm sensor board on the front of the turbine. Simply wipe a refrigerator magnet (really low power magnet) across the face of the sensor. That usually restores it to normal. The Halls are very sensitive and can be magnetized easily by spurious signals and tools coming close to it. I had one customer that would screw on the sensor with a really strongly magnetized hex wrench and screw up the sensor every time!

Len Gladstone