a update on my bad MPG i cleaned all the 02 sensors the other day. the 1 behind the cat was filled with material from the cat so i cleaned it all out after that i decided to clean them all the other 2 seamed good then i checked B1S1 that 1 seems fine but it has a rattle when i shake it but nothing came out? no codes or anything seems to be working. today i filled up and did my math and now im getting about 16 MPG i think it might get a little better when summer comes and this crap winter gas goes away.
do you think i should replace the 02 that's rattling? i don't think they have been changed 221k miles

 

Oxygen sensors react to changes in fuel mixture from lean to rich, and rich to lean. The faster they react, the better the fuel mileage. As the sensors get miles on them, the reaction time slows down. At ~100K miles, they have slowed enough to make a substantial difference in mileage and performance, without setting a DTC and turning on the SES light. That's why the factory recommends replacing them. If you wait for them to turn the light on, it's gonna cost more in fuel. AC Delco and Denso sensors have the fastest reaction times. Denso sensors are a little less expensive, but they don't last quite as long as AC Delco. B1S1 and B2S1 monitor fuel mixture. Sensors after the converter(s) only monitor the converter, and do not affect fuel mileage. Those you can get away with waiting for the SES light to come on.

 

should i replace both B1S1 and B2S1 and leave the other 2? what the sensor be4 the cat do?

 

I remember when gas went from $.52 a gallon to $1.02 me and my friends had a heart attach. Less beer and pot money. 32 bucks a lid.


Change both, 221K you have spent soooo much extra in fuel, at todays prices you will start putting extra $$ in your pocket in a week. Like Chuck said just worry about the front ones.

 

Have you ever checked the EGR Valve?

 

Ditto to spittybays reply.

Scott, don't know what year yours is but....
This is how the oxygen sensors are arranged on 96 & 97:







98 and newer do not have the B1S2. On 96 through 2005, sensors B1S1 and B2S1 constantly monitor the fuel mixture. The PCM adjusts fuel mixture according to what the sensors report. If a sensor reports lean, the PCM adds fuel until the sensor reports rich. If a sensor reports rich, the PCM decreases fuel until the sensor reports lean. As the sensors get old, they "default" to a lean output. The PCM richens the mixture trying to make the sensors report rich, but the sensors are incapable of reporting a rich mixture. So the PCM keeps adding more fuel. As a result, crap gas mileage, crap performance, and eventually the rich mixture destroys the converter.

 

Thank you all ill replace the 2 sensors
i have a 96 with the 4 sensors just like the diagram you posted