They do not do visual tests under the car. However, EVEN if you clear CEL's before smogging the car you will more than likely fail the sniffer test everytime. Some people I know just go stock to smog and then go home and put HFCs or test pipes on for the next 2 years. Don't go looking for a shop that will do **** under the table because the state has cracked down big time on that and if you get busted you will get a hefty hefty fine