I have a problem with honda fireblade cbr900
I have the neutral bulb is always on! Any suggestions please?

 

What model of bike? (This will determine whether it is switched power or switched earth and how many connectors we can expect on the switch).

Either way the most likely answer is a faulty switch, if we assume that the bike is switched earth with a single wire to the switch assembly (the normal) then it could also be a short from the switch wire to earth.

Disconnect the switch wire and use a multimeter in the lowest ohms range to measure the resistance from the switch connector to a good earth point whilst in neutral and whilst in gear. In neutral it should be effectively zero and in gear it should be open circuit - it sounds like you’ll get zero in both cases in which case you need a new switch.

If the switch is good then measure the resistance from the connector to earth (ignition switched off). This should be open circuit, if it is zero then you have a short and we’ll have to trace that back.

 

Yup, bad neutral-switch or its wiring shorted to ground.

Although you may find it’s intentionally shorted to ground at connector due to POs desperate attempt at fixing no-start problem.