I found out why the new sensor was reading “error”. The plumber had connected it to the wrong point on the main board! It is curious that the system should work better when a sensor isn’t connected than when a faulty sensor is connected.
Anyhow, I rewired the sensor to the correct point and everything is now working fine. It only required 4 plumber visits to diagnose the problem, change the sensor and wire it up incorrectly!
Hopefully, I can enjoy my shower in peace now!