HP 5Si laserjet - incorrect error message

Hi there,

I don't know whether someone can help me with this but here goes.

We have a very old HP 5Si Laserjet printer which HP unforunately don't
support any more.

We've started getting a message coming up telling us to empty the top output
tray even though there is no paper in it. It doesn't happen all the time
but it has started to happen far more frequently over the last few weeks.
Does anyone know of a way of disabling this so it can continue printing, or
alternatively how do I get to the actual sensor so I can disable that.

Any help would be much appreaciated

thanks in advance

David