Q: Why does my Nissan 300zx lose power?

Hello, we will be happy to help. I’m curious to know why the guy told you that you needed a new oxygen sensor and fuel pump. I wonder if the oxygen sensors were exhibiting that of lean conditions. If the condition was lean that basically indicates that there’s not enough exhaust gases present or there’s too much air and not enough fuel in the exhaust gases. This condition can be caused by a weak fuel pump, a misreading oxygen sensor or even a clogged catalytic converter. A clogged catalytic converter is most likely your problem, the air can’t escape the engine so it will basically be choking while its trying to exhale. You’ve already replaced the other likely causes so the next thing I will look towards is the catalytic converter. A technician could check the flow of the catalytic converter by loosening it from the exhaust and inspecting it for restrictions. Or a little more fun and more efficient way to do it would be to take the catalytic converter down and go for a test drive to see if that solves your problems. If you start the car with the catalytic converter off it will sound like a racecar so cover your ears! Good luck. If you need any physical assistance you could schedule one of the mobile technicians here https://www.yourmechanic.com/ to help repair your vehicle. Also I recommend to check the performance of your clutch. A slipping clutch could cause that loss of power feeling too.