O2 sensor

How do you know which O2 sesor is bad, and anyone know a reliable place to get them?

very poor gas mileage could mean a bad O2 sensor. autopartswarehouse.com or bavauto.com are where i get my parts.

a bad O2 sensor

will usually throw a check engine code.

what vehicle do you have?

I have a 1995 M3, there are 2 (pre and post cat) O2 sensors correct?

i have a 1990 325i.i believe i only have one O2 sensor

No. Only one O2 Sensor on the 95 M3. Nice thing about the OBD-1 cars.

for my 84 325E I used a generic 3-wire sensor and just spliced it in. This one (a Denso) was $25 on evil bay, but I've used one like it (but Bosch) on my 93 SAAB for $60.