[Mr Gazza]

Is there a way of testing the sensors off the bike (or on it, if relevant).

For instance if you dunked the water temp sensor in some hot water of known temperature, it should register on the dash, no? (presumably it would need to be earthed to work?) That particular sensor probably doesn't talk to the ECU, but you see what I'm getting at.

If the sensor was sending wrong information, this would not register as a fault because the ECU could not check the info, just read and accept it.

One scenario I can think of that might hinder starting is, if the ECU thought that the engine was still hot from a "stuck" temperature sensor, then it would not engage a cold starting mixture. Sparks and fuel would still be in evidence and no fault code would register.

Detecting fault codes is the first call though, but if this still presents an impasse then testing sensors might be the way to go.
If they are cheap this could be simply done by substitution, one at a time.

Does anyone know if any or all of the sensors can be bench tested at home?