I have a 1995 volvo 850 t5 and i'm thinking of replacing the whole exhaust from the turbo and the plan is to try it without the cat' first but how will the second o2 sensor react?

 

Volvos use 4 wire O2 sensors - which have a heater and a sensor (the sensor needs to be around 600 degrees to read properly) so if you are thinking of simply doing without the sensor, you'll get a check engine light with codes. if you don't plug the sensor into the pipe, you'll had a little hot item to contend with.. The readings from the sensor also feed into the computer to adjust the mixture as well so that's going to impact performance.