I have a 2006 Lexus IS350 that has always run fine until a couple of months ago when it started experiencing shifting issues. Delayed shifts and jerky gear changes. When launching at full throttle from a stop, the car initially pushes forward like it wants to go but then slows down and refuses to change gears. it just revs to around 4-5k rpm when full throttle and stays there. when driving conservatively that car shifts ok. Since no check engine light or error codes appeared, I took it to a mechanic. The initial advice was to service the transmission and check the catalytic converters, which I did but the problem persisted. At this point, I decided to replace the entire transmission with a used one from a trusted parts supplier. During the replacement, the mechanic also noticed that one of the headers was missing a catalytic converter. not sure if my pops and bangs tune caused that However, after installing the replacement transmission, the issue still remains exactly the same. I'm at a loss for what to check next. Has anyone experienced something similar, or does anyone have suggestions on what else could be causing this? you guys are me last hope

 

Hello, how many miles does your IS have?
Another question is why your car has one of the headers with a cat and the other is catless?