I'm trying to troubleshoot an error with an HP ink jet printer (Photosmart
3210) that says "Ink System Failure". All other components seem functional.

In the printer there is a small PCB with a few soldered components that
appear to sense ink flow or pressure. It is in series with the pump output.

http://i48.tinypic.com/o8xz84.jpg

http://i49.tinypic.com/r76kib.jpg

How does this work? Are these 6 LEDs and photodiodes? Just detecting the
presence of ink doesn't seem likely. Wouldn't movement or pressure be what is
detected?

What does this PCB do and how does it do it?

Thanks.