95 blazer 4wd 4door. how can i tell if my 02 sensors bad? what bout a fuel filter? getting less than 15mpg per gallon and i did tune up expected a fuel filter. i hear that people miles increase after changing the sensor. i dont know why in getting in the low teens. How do people get close to 20mpgs?

 

Try changing your fuel filter.
The o2 sensor will generally throw a code. Do you have a check engine light on?

 

An O2 sensor could need to be replaced without throwing a code. Sometimes they get lazy and don't report back as quickly as they used to. I'm not sure if there's an electrical test you can run on them to see how often they send out the info but that would be a good way to test it.

 

you can pull them out and put a torch on the tip while reading voltage on the output pins with a DMM. It should fluctuate quickly.