I have been getting an SES code (420) for a bad catalytic converter. Just to make sure the cat was bad before I replaced it, I replaced the O2 sensor.

Now, my gas mileage has dropped from 24 MPG (average city and highway) to about 19 MPG.

Any ideas why the mileage would drop after a new O2 sesnor?

You replaced the downstream (after the cat) O2 sensor and you got a drop in gas mileage? The downstream sensor has no impact on the fuel delivery. It only monitors the catalytic converter.

It is possible that your cat is going bad. If so, I believe that GM warranties their cats for 8yr/80,000miles.

Good Luck.

I replaced both sensors (before and after the cat).

The reason I replaced them is because I had an SES light with a code of P0420, which is a low catalytic converter efficiency code. Before I spent the money to replace the converter, I replaced the sensors, just to make sure the problem was not a bad sensor.

I reset the code, but it came back yesterday, which means the converter is bad. I just rolled over 100K, so the warranty on the cat is gone.

But, that does not explain why the mileage dropped after the sensor was repalced.

What brand sensor did you use as replacements? Borg Warner are absolute crap and Bosch just don't last. AC Delco sensors are the only ones that I would use and are the only ones I recommend to those that I help diagnose and repair.

Many muffler shops have a back pressure analyzer that they screw into the O2 bung pre and post cat. They'll do a free diagnostic to tell you if the catalytic converter is bad.

Now does your exhaust smell like rotten eggs? Most of the time when the converter takes a dump (or clogs), the exhaust smells horrible. Also, do you notice any bogging at higher RPMs that would be as a result of a decrease in flow through the converter? I haven't really heard of converters just not burning the excess fuel with no other symptoms...

The sensors are made by Denso. I Bought them at a parts dealer that is well know in this area for high performance mods, so I don't think they sell anything that is crap. They did give me the option of this brand or Bosch, which was about $20 more, so I went with the Denso.

The only other noticable problem is sometimes it will lurch on the highway when I give it gas after coasting.

I did take it to a Meineke Muffler shop to see it they could test it, but as I thought, they did not have any test equipment. They just banged on it and listened for rattles, which it did not have.

I don't think there is any question that I need a new cat. I just want to make sure I have corrected the problem that caused it to go bad in the first place. I had a bad plug wire about 8 months ago, which caused some serious misses and I could not fix for about 2 months. This was probably the root cause of the cat failure.

I now have new plugs (NGK), wires (Belden) and O2 Sensors. Hopefully, this takes care of any problems I had.

This drop in gas mileage has me puzzled though. A new sensor should not have caused a drop in mileage, even if the cat needs to be replaced. I guess the only way to know for sure is to wait until the new cat is installed and see how the mileage goes.

Did you reset your computer during the install of the new O2 sensors? Actually, whether you reset it or not, the PCM is going to have to relearn the LTFT (long term fuel trims) in order for it to get the best fuel mileage anyway. Resetting the PCM will help it out because it'* starting from a zero point, but without the reset, you have no idea where it'* working out to be. Give it acouple of tanks and see what it is then. If you are sure that your cat is bad, then it'* time to replace it. Without atleast scanning the vehicle, I can't say whether it does or not. Nor can I say that it'* the O2 sensors fault or another part of the system.

Also, just because one particular sensor works well on one car doesn't mean that they will work well with yours. Bosch sensors run great and have a long life in Mopar vehicles. I replaced my sensors in my Intrepid when I bought it and they worked great for the 4 years and 75,000 miles until I sold it. However, the same sensor in a GM vehicle may crap out in 1-2 years. The quality of the sensor is what matters the most.

When you say reset my computer, do you mean disconnecting the battery?

Yep, disconnect one pole for 30 minutes and then replug it.

edit: Probably your cat, i replaced mine with a high flow one, and gain good power,
if you plan to add power, i recommend getting a high flow cat.

Wow. For the price of those two sensors, you could have purchased a performance cat.

Buy AC Delco sensors ONLY. Bosch are absolute crap, and I don't think anyone here has ever run a Denso.

My suggestion at this point is a catalytic backpressure check at your friendly neighborhood exhaust shop.

Never change the rear O2 unless you get an emissions light caused by the sensor itself. As stated, it has no effect on performance or efficiency whatsoever.

I am taking it to a muffler shop tomorrow to have the backpressure tested. This shop has the equipment to do a test (unlike the Meineke shop I went to first).

If I have a high flow cat put on, will I need to get a special rear O2 sensor?

By the way, the sensors only cost $95.00. It was worth a shot to see if the sensor was bad and would not have to replace the cat.. And yes, a bad rear sensor can cause it to throw a low cat efficiency code (confirmed by the dealer). The front sensor was black, so that needed to be replaced anyway.