1995 Toyota Avalon Check Engine Light displayed

Check engine light has displayed for at least 2 months, My mechanic tested it and reset it and did some repairs. Looking at my receipt looks like parts used were amplifier and valve ($72 and $64 respectively with labor of $160 - making total bill $303!). He finally advised me to go to the dealer(after checking car again) because the Check Engine Light lit up the dashboard again. I'm afraid to do so because dealer wants $120 to hook it up to some machine. Please help.