There's nothing on the chip which isn't visible to you on page 31 of your passport:
http://www.passport.gov.uk/general_biometrics_ chip.asp
The US authorities have no direct access to UK criminal records. The data from the chip can't log into a system which reads your record because no such system exists. Scanning your passport will only 'start alarm bells ringing' if your name is on a 'watch list' based upon specific information passed on by the UK authorities. For security reasons, we're not told who's on this list but it will almost certainly contain details of anyone who's been convicted of serious crimes involving drug trafficking, arms dealing and serious sexual offences. The list
might include people convicted of more minor offences (simply because the UK
might decide on a 'blanket policy' of passing on information, say, about all drugs offences or, perhaps, all sexual offences). However, it's extremely unlikely that, say, minor theft or criminal damage offences would show up.
Chris