yo so i bought a 97 z34 for 1800 bucks last week and now after i have cleared the DTC codes and found some massive vacuam leaks, now the car has no power after 3 grand at wot, the DTC codes it was throwing was cylender 5 missfire, 02 sensor low voltage, and engine cooland temp sensor fault, well the engine colant temp fault was due to the plug being not pluged into the block, so got that taking care of and cleared hte faults, but now it acts like the leans out huge now at around 3k, im thinking a cat or a 02 fault. but fiqured id ask here. thanks

 

I would lean towards the cat. How many miles on the car?

 

120k, but i did notice that today when i did a scan on it, i was seeing no knock at wot, and the 02 was reading 0 volts over 3500 rpms.

 

Try the O2 sensor first, it will be cheaper than replacing the cat. If that doesnt fix the problem at least you can rule the O2 sensor out.