code 55 says this: 55 - Other Malfunction. CONSULT Terminal Required
i dunno if it's worth it to you to look at this, but try going to this link:
http://lib.store.yahoo.net/lib/phase...s14sr20det.pdf
and go to page 208 and follow the flow chart. you'll want to do the steps that show an "x" going across the little picture of a scanner (since you probably don't have access to a consult).
also since you haven't checked the ho2, try checking out page 338's heated oxygen sensor section to check the resistance of the ho2. if i understand page 194 under ho2, it looks like the o2 sensor is supposed to fluctuate voltage from 0v-0.3v to 0.6-0.9v when cross counting. if you have access to an oscilloscope you could check that as well and see if it's in range.
other than that, might just try starting at the beginning of that section and going through the flow charts and trying stuff. without access to a scan tool to see input/output displays, can't really get a good idea of where to start. but since you said unplugging the o2 "seems" to help it, that's where i'd maybe start. oh and that fsm should work for either s13 or s14 sr20det's per p2m's site.