I recently installed megan test pipes on my G35 (downpipe with cat removal) and the check engine light came on due to the O2 sensors being disconnected. However, when I started driving it and giving it a bit of gas/going up a hill and pressing the gas harder, the car began to shake back and forth a bit. I’m not sure if it’s because of the downpipe or if it’s something else that happened to occur at the same time I installed it, but I just want to make sure. I also took it back into the shop, no leaks or loose bolts, but there is a manufacturer specific code when scanned. Anyone know either the reason the car is shaking or the manufacturer code?

If you have the O2 sensors disconnected then it won't run right. It can't give it enough fuel all the time. Reconnect the O2 sensors then go through the throttle relearn and idle air relearn.

I second this. They must be plugged in even if reporting signal outside their OEM range.