Hi. My 97 Accent GT is running just fine, and only has 55k miles on it,
but I took it to get it smog checked in CA the other day and the guy
said it was failing so he shut it down before it reported the failure
to the state.

He said that because he shut it down he has no idea why it is failing.
Suggested MAYBE if I get the oil changed it might help.

My question to you experts is:
What should I do or what can I do to help it pass when I take it in
next week?

Someone else told me that getting an oil change andt tune up will not
help. I already ran a dose of engine cleaner in it, and that apparently
didn't help.

I was very surprised when it did not pass.

How should I proceed? What I am hoping to avoid is a situation where I
take it to a mechanic after it fails and they run up a $300-X000 bill
getting it to pass the test. It really should not be in bad shape with
the number of miles I have on it, and with it running good, not burning
oil or anything...

Thanks for any suggestions.