Automatic Transmission problems of the 1997 Nissan Maxima

It all started with the noises from the exhaust. So I fixed the flex pipe and no more noise, the mechanic said it's a common Maxima problem. Then the check engine light came on. So I fixed the O2 sensor and the knock sensor, the mechanic said it's a common Maxima problem. I drove the car to the inspection station a few days later and the light came on. The knock sensor code came on, the O2 sensor, the vehicle speed sensor, there was an egr malfunction, a serial a/t link communication malfunction and possibly other codes I cannot recall right now. The mechanic said it's all common Maxima problems. I've spent so far at least $800 and the car is still not working properly, and I cannot pass inspection.