HP 1220c inkjet error code 90020403 ?

Hi

I have a 1220C inket that most of the time refuses to work. Sometimes after
a few hours (literally hours) of sitting there (sometimes moving the print
head back and forth - sometimes buzzing) it will decide to work again.
During its latest working session I printed the diagnostic page (hold power
on and press feed 4 times) and it shows the last error code as 90020403.

I can't find any trace of numbers like that on the net. Does anyone know
what it means?

The buzzing comes from low down near the paper tray at the service station
end. It is NOT the service station motor. I replaced that already. When it
feels like printing the service station functions fine. Carts are new and
contacts are clean. Its 3+ years old but has only printed less than 4000
pages.

Any help appreciated

Robert.