2001 Toyota Rav4 won't pass California smog test, no engine light on.

Our 2001 Toyota Rav4 won't pass California smog test, no engine light on.
We have taken it to 3 different mechanics and the local dealer, and spent a
lot of money, but no one can figure out why it won't pass the smog test.
Hoping someone has experienced this and has some ideas. With a different
car, a check oil light turned out to be a loose connection, hoping to find
something like that for this problem. Thank you for your help!