I have an '00 chevy with the 4.8 in it that I have been battling with the service engine soon light on. It has had a flowmaster exhaust (still single)added from the cat back and I believe this is causing the issue. The code reads that the O2 sensor (unsure which 4) is causing the trouble and I had them replaced behind the cat but I still get an intermittent SES and it's caused my mileage to go down to almost 12mpg on the last tank. Could the cat back exhaust be causing the code b/c of too much airflow? I've seen O2 sensor "plugs" for sale that give a false correct reading for stuff like catalytic elimination......would these work and if so....where can I get some cheap ones. Any suggestions would be great!

I have an '00 chevy with the 4.8 in it that I have been battling with the service engine soon light on. It has had a flowmaster exhaust (still single)added from the cat back and I believe this is causing the issue. The code reads that the O2 sensor (unsure which 4) is causing the trouble and I had them replaced behind the cat but I still get an intermittent SES and it's caused my mileage to go down to almost 12mpg on the last tank. Could the cat back exhaust be causing the code b/c of too much airflow? I've seen O2 sensor "plugs" for sale that give a false correct reading for stuff like catalytic elimination......would these work and if so....where can I get some cheap ones. Any suggestions would be great!



Reset the computer and see if it sets the codes again. The exhaust shouldn't cause the CEL unless you damaged one of the o2's or it just went bad. If you read the code it will tell you which one it is.

Eample.. P0133 - Oxygen Sensor Circuit Slow Response (Bank1, Sensor1)

The better way to fix that like if you were to not have converters or whatever, would be to get somebody with HP tuners or Edit and turn them off. (you can only turn the rear off)

Eample.. P0133 - Oxygen Sensor Circuit Slow Response (Bank1, Sensor1)

Sould like your cats are bad if you replaced the rear O2's and still get the SES light.

Is it both rear O2's or one or the other. POST the code if you want help!!!!

It's hard to get a read cause the SES intermittently comes on and is not on at the moment, but I'll give it a day and it will go back on

It's hard to get a read cause the SES intermittently comes on and is not on at the moment, but I'll give it a day and it will go back onEven if the lights not on the code should be in the computer! Scan it and see.

Even if the lights not on the code should be in the computer! Scan it and see.


X2 if the light was on OBDII vehicles store the code even if the MIL is off.

Post the DTC.

I have an '00 chevy with the 4.8 in it that I have been battling with the service engine soon light on......................The code reads that the O2 sensor (unsure which 4) is causing the trouble........................................... .......]


.............................[ but I still get an intermittent SES and it's caused my mileage to go down to almost 12mpg on the last tank.



If you had a real scanner it would tell you which one instead of guessing





If this is the case then it probably is a front sensor. A rear sensor is going to tell you that the convertor is more than likely dead.

Agreed with all the above. The cat-back exhaust shouldn't cause an O2 code, the rear O2's basically just monitor the cat efficiency and therefore wouldn't make any difference in how the engine runs. The O2 "plugs" you talk about are just for people who think they need to cut the cats off and bascially fool the computer to not turn on the SES light, that's all.

Things that could cause the light (there are many, but just a few ideas):
- the sensors or harness were damaged when installing the exhaust system (figuring out the exact code - exact sensor - will narrow this possibility).
- if it IS a rear O2 sensor code, as mentioned the sensor itself won't affect performance, but it could be telling you the cat is bad and it (the cat) could cause the mileage issues if it's plugged.

Agreed with all the above. The cat-back exhaust shouldn't cause an O2 code, the rear O2's basically just monitor the cat efficiency and therefore wouldn't make any difference in how the engine runs. The O2 "plugs" you talk about are just for people who think they need to cut the cats off and bascially fool the computer to not turn on the SES light, that's all.

Things that could cause the light (there are many, but just a few ideas):
- the sensors or harness were damaged when installing the exhaust system (figuring out the exact code - exact sensor - will narrow this possibility).
- if it IS a rear O2 sensor code, as mentioned the sensor itself won't affect performance, but it could be telling you the cat is bad and it (the cat) could cause the mileage issues if it's plugged.


X2 on the wiring. My wifes Blazer was throwing a code for the rear o2 sensor, so I replaced it. As soon as the drive cycle was completed, the code and cel came back. I traced the wiring and eventually found where it had come into contact with the exhaust pipe (under the vehicle, but closer to the firewall than the cat). The loom was melted, and the wires were heated enough that the 02 heater wire had become brittle and broke. I repaired the wire and haven't had another code since.

You need to get the DTC and hopefully some snapshot data from when the code was stored. Not knowing which one of the expensive sensors is faulty doesn't do you any good.

O2 sensors are probably the most commonly incorrectly replaced component on a vehicle. The codes are getting better at telling you what is wrong (i.e. heater circuit error, slow switching, etc), but the code refers to a problem in the circuit. That means it could be the sensor...or it could be the wiring...or it could be the controller...or it could be something else.

For instance...a sensor that is switching too slowly or is giving readings the computer thinks don't jive could be something totally unrelated. Just recently I had to work on a Grand Am with a 2.4 Twin Cam (:barf:). The guy was convinced it just needed an O2 sensor. The codes stored were for an O2 not switching fast enough and a faulty CTS indicating too cold. Well I hooked up the scan tool and noticed that the IAC counts were very low and the LTFT was in the neighborhood of -20. I grabbed the propane torch and isolated the problem to being a failed injector o-ring acting as a vacuum leak. The computer was setting fault codes for the O2 because the IAC was compensating for it being too lean. Meanwhile, the LTFT went so rich that the computer also thought the CTS must be reading incorrectly (even though voltages were correct) because it was going so rich.

The point is that everything is interrelated. The computer is smart, but only as smart as the programming. If something outside the realm of what is programmed into the look-up tables occurs it will do its best to correct the issue and diagnose the problem, but it isn't perfect. You need to diagnose the problem rather than look for a silver bullet part replacement unless your wallet is a lot more padded than mine.

*edit* The vacuum leak also caused a horrible drop in mileage due to running so rich. But it ran OK. The exhaust smelled and it chugged a little at idle, but it wasn't horrible. The computer is able to compensate for some serious problems while still retaining decent driveability. That's just some food for thought.