Which O2 sensor is most important?

[postmortal1]

Just out of curiousity. Which O2 sensor is the one used primarily to adjust the Fuel Injection/mapping??? The front non heated or the post cat heated??? on a 97 DOHC.

[ruley73]

Originally Posted by postmortal1

Just out of curiousity. Which O2 sensor is the one used primarily to adjust the Fuel Injection/mapping??? The front non heated or the post cat heated??? on a 97 DOHC.

The upstream sensor(s) on any car are the most important and are used by the PCM for calculating fuel delivery. The downstream sensor(s) is/are only there to make sure the catalytic converter(s) is/are working and nothing more.

[postmortal1]

So if a guy was limited to changing only one of the O2 sensors, the one on the header is the one to replace, which will then potentially pay for itself in MPG gains???

[ruley73]

Originally Posted by postmortal1

So if a guy was limited to changing only one of the O2 sensors, the one on the header is the one to replace, which will then potentially pay for itself in MPG gains???

Assuming the one they are replacing is worn out, yes.

[postmortal1]

Well I had the code for O2 heater (p0141 it think) a few times and just recently got the slow O2 (0137 code). This is a new 97 SW2 with 115k 94 DOHC (28/30MPG) project car and am not getting as good of MPG as i got with my 1 owner 230K 91 SL2 (33/34MPG) and am attempting to resolve that.. Plus there is no way in hell I am getting the post cat O2 out of the rust ball mount.

[DonP]

The post cat (rear) O2 sensor is a tell tale for the emissions inspections. It serves no fuel control purpose. P0141 and P0137 are both rear O2 codes, heater circuit , and low voltage respectively.

The front O2 does control fuel mix and if it is getting slow you can count on decreased fuel economy. P0133 is the slow front O2 code.

[eRic 02sc2]

Originally Posted by postmortal1

Just out of curiousity. Which O2 sensor is the one used primarily to adjust the Fuel Injection/mapping??? The front non heated or the post cat heated??? on a 97 DOHC.

the front oxygen sensor in the exhaust manifold is "most important" because it is used for fuel control. the rear heated oxygen sensor is only used to monitor the operation of the catalytic converter.

[signal7]

For what it's worth, I just replaced the cat on a 98 SL and I did manage to get the rear O2 sensor out easily. Once the cat is off the car, you can carefully heat the area around the O2 sensor bung with a torch. Do not allow the flame from the torch to hit the O2 sensor or you could damage the sensor. Once the pipe is good and hot you should be able to get a wrench on it and unscrew it.

... or you could just buy the replacement. I was replacing the cat anyway, so having to take the O2 sensor out of the old cat wasn't a big deal. It only took me an extra 10 minutes of time, but in your case it's a much larger job.