Mileage is 168K miles, 6cyl, automatic. I bought this car about 3 months ago and don't have any maintenance history. I'm doing my best to convey what my mechanic told me.

It is showing a code that it is running lean (sorry, I don't have the code number). But, he said it is running rich. The long term fuel trim is showing as -20. He believes the O2 sensors have been changed and the MAF sensor is also not original. He created a vacuum leak and it didn't change the fuel trim number. He also said he added fuel to create a rich running situation and that didn't change the fuel trim number either. He said the plugs are getting fowled and that's why I was getting sluggish running during acceleration. He knows I don't want to put a bunch of money into it and it not confident enough to know what the issue is to want to try to something. He said his best guess is that it may need a computer. He suggested I may want to take it to the Lexus dealer because if it needs a computer, he thinks they may have to program it.

Prior to this I was getting misfires in cylinders 1, 3, 5 (think). They were the back ones that are laborious to change. He replaced all 6 spark plugs and also replaced the valve cover gaskets as they were leaking.

Thank you in advance for reading this and I appreciate any suggestions you may have.