No cat, running RICH

'05 3.8 series 2. So someone cut off the cat while it was parked on the street, and so in the mean time while waiting to replace the cat, it’s got a temp pipe and is now I find it’s running super rich. Was 21 MPG now 13MPG and exhaust smells - wow. Rich.
I expected that when the cat was gone, the car would go into a limp mode while it’s computer tried to figure out what was going on, and that limp mode wouldn’t be so rich. Is that normal or should I be looking elsewhere for the cause?

Is this the vehicle that you had the Catalytic converter stolen from in August of 2022 ? If so why have you not fixed it or do you just want to pollute the air ?

Sounds to me like you need to replace the Catalytic Converter… If you have exhaust leaks around your air/fuel ratio sensor (upstream) that will confuse the computer and with the cat missing, your O2 sensor (downstream) is really going to be off with the cat missing…

Or you could go to a dyno/tuner shop and spend big $$$ to have a tune installed to eliminate (trick the computer) the converter, but you still can’t have any exhaust leaks above the O2 sensor… If they even can on your vehicle…

Thanks - good to know; maybe I can meter that upstream sensor out to verify it’s function. No exhaust leaks there that I’m aware of but perhaps the reduced back pressure is causing an issue. Replacement is going to be a nightmare - dreading the effort involved. Meanwhile it’s got other issues I’m deciding if it’s worth it. Certainly not worth the ROI with tuning or tricking the computer.

Your local exhaust shop will have NO trouble installing a new Cat for you…


No big deal to have a new catalytic converter installed, unless in California. There, cats are more expensive, same cat, but to my understanding, have to be certified by the state.

No, that is not the issue. The “lack of backpressure” meme is a myth.

If you scan out the OBD codes, it will tell you what the computer thinks is happening. I’d guess it IS in limp mode… but if it shows more than just a P0420, which I expect it will, at least you know where to start. Anything else is a wild donkey guess.

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Downstream 02 sensors, just monitor cat efficiency, they don’t have any input as to how the ECU runs things. The upstream 02 sensors are the ones that matter as far as how rich/lean things get. That’s why you can (or used to be able to) turn off the rear 02 sensors with a hand held tuner and the car would run fine.


Thanks, this was my original understanding of it, so when the car went so rich around the time when the cat left the building, I mentally correlated those issues. It’s quite possible they do not correlate though (I’m not the driver). Last time I scanned it, it didn’t have codes other than the downstream but I’ll rescan and see if anything comes up now. I know how much the cat’s going to cost - but I’d like to know about all the other expenses I’m looking at here.

Thanks - Wasn’t sure with the location so close to the area in question. On my diesel the BP sensor is used to calculate the VGT % and I wasn’t sure what, if anything, and if it existed, it would be used for in calculations on the 3.8.

I’ve heard that before, but I’ve also heard that on some engine configurations the downstream sensor is also used for rich/lean calculations. Are you speaking in general , or just this particular engine?

As far as the OP, I’m guessing there’s a small exhaust leak allowing fresh air to get sucked towards the o2 sensors. Sometimes it is helpful to have an assistant partially obstruct the tail-pipe, piece of wood or something, makes finding an exhaust leak a little easier.

Try disconnecting both O2 sensors. It would certainly better than what’s happening now. At least the down stream sensor as it has no useful function now.

Did any wiring get damaged?

Every 96 or newer car I’ve had or have mechanical experience with is that way (downstream sensors just monitor cat efficiency and nothing else). It’s certainly plausible that there are cars where the downstream sensors do more than that, but I’ve never heard of any specific ones that do.


When the cat burglar strikes, they often do more damage than just cutting out the cat, they often cut through wires, vacuum hoses, vapor vent lines etc. You have more issues than just a missing cat.

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Oddly, the ONLY codes it had set was EVAP and coolant temp too low. I expected to see something about the O2 sensor. The car’s temp gauge reported normal in the middle of the range and Torque Pro reporting it to be 199F, but maybe there’s something intermittent going on. I’ll replace the sensor and the thermostat while I’m there.
I did find the upstream O2 sensor properly plugged in. I unplugged it as a test but this far have not noticed improvement.

I’d expect that too, but it might take several drive cycles before an o2 sensor related code pops up as a current code. Each drive cycle may require certain things to happen, fuel tank may have to be in certain range, a certain number of miles driving in speed range, etc. Are you able to see pending codes w/your scan tool?

The first code you see may well be for the cat itself. The computer compares the signals from the pre & post cat o2 sensor, expecting to see a certain relationship, signals will be different. With no cat the signals will be more or less the same and that test won’t pass.