What I’d do is go to AutoZone or someplace similar and buy a cheap Scanner that will read and clear your codes … And a gascap … or just a gascap gasket if your mechanical skills extend to prying the old gasket off and slipping the new one on. The Scanner will cost about $70 give or take maybe $10. If you buy one that does CAN as well as whatever your Toyota uses (probably ISO-9141) the scanner will probably work with your next car as well.
I’d then replace the gas cap and clear the code(s) using the scanner. If you are lucky, the problem was due to air leaking past the ancient gas cap gasket – which the diagnostic system can’t distinguish from a hole in the gas lines and the code won’t come back.
If codes do come back, research them and decide what to do. A caution – there are a number of codes associated with the evaporative emissions system. They are not noted for being especially accurate about which component has failed. Other than the gas cap which should be dirt cheap, you would do well to avoid financing a parts hanging expedition with your mechanic serially replacing parts until he finds one that makes the problem go away.