I have a 99 volvo v70 xc and lately its been sputtering bad when stopped and spitting black smoke from exhaust. Also has horrible gas mileage. I think its the o2 sensor but which one and how do you tell which one is bad. I got codes read today and 3 came upin the following order
1-p1171
2-p0171
3-p0135
Any tips would be greatly appreciated.

 

Looks like the front O2 sensor is bad.