Check engine light that never goes away! '99 Lexus ES300

I took my car to inspection and failed the emissions section with 4 codes. I narrowed it down to 1 being: P0107. I’ve taken it to the mechanic and had the sensors, fuses, and gas fuel regulator replaced. After each visit to the mechanic, the check engine light would be off and the mechanic told me to run it for 20 miles before having it reinspected. After driving for 10 miles or so, the check engine light reappears and I would bring it back to the mechanic (four or five times already). I don’t know what to do anymore. Help!!

You need a new mechanic. He should know that when he resets the CEL, that the computer has also reset all of the system monitors. These monitors are like little programs that run in the background, checking to insure that the emission systems are operating properly. No emissions test will pass you until these monitors are set to ready, even if the CEL is not lit. The requirements vary between monitors but typically they require several start cycles and varying driving conditions. 20 minutes of running won’t do it.

He is resetting your CEL but not fixing the underlying problem. To minimize false alarms, the computer requires that the problem be detected a certain number of times or that it exists for a preset amount of time before it actually turns on the CEL.