99 Gt..Problem w/ 02 Sensors

First what is the engine code number? The removal of cats will make a stock tuned computer think that the cat is not working and will throw mulitple codes including saying it is running rich. Running the car too rich will give it the misfiring feeling in some cases. I suggest looking up the code numbers (it would be like p0420 or something like that) and telling us.

And I wouldn't replace an o2 sensor until you either had the car tuned to turn off obd2 monitoring or got mil eliminators for those back o2 sensors.

Exactly what he said above me.

dont rush out to replace the O2 sensors as they are not cheap to begin with. You can go by Autozone and have the parts guy come out with a scan tool. Its totally free and they will print you out a small receipt with the codes listed.

You cant just take your cats out without doing O2 simulators, custom tuning to delete the rear O2s or something.
Not getting on ya, just trying to help.

Let us know how it goes.

Those codes would be really helpful. It sounds like you've been throwing parts at the symptoms so far instead of getting to the root problem(s).

Forget about the lack of cats and the rear O2 sensors throwing codes - they have no effect on the tune, they are only there to monitor the cats' effeciency. The computer only uses input from the front O2 sensors for adjusting the air/fuel ratio. MIL eliminators or turn them off with a tune will get rid of the nuisance CEL being on all the time, preventing you from knowing when you've thrown a code you do need to worry about.


It is very unlikely that 2 O2 sensors would fail at the same time unless something else killed them, like running really rich long enough to foul them. A bad sensor should only affect its own side of the engine so ifit was causing it to run rich it would only be rich on that side so it shouldn't be able to foul out plugs and ruin the other sensor on the other side of the engine. If all the plugs were fouled then you need to look for something that can throw off the A/F ratio for the whole motor - vacuum leaks, MAF and other sensors, again those codes would be handy - if something is preventing the computer from getting into closed loop operation or is putting it in limp mode then you'd be running rich.

Slo99Vert said:

Hey thanks a lot for your help! Actually I went to the Ford dealer, and they told me the same thing that advance did..that it was bad 02 sensors (that was the reading from the diagnostic codes). But anyways I replaced 3 of the o2 sensors and the car is running like new now. Took it to the track last night, didn't misfire any! I just purchased the 02 sensor things(sorry I'm a girl and don't know the correct term) for the rear where it tricks the computer into thinking theres cats..So hopefully this will solve my problem. But thank you all again!

Click to expand...

The sensor things are called 02 simulators.

It's really odd to have multiple 02's go bad at once. If this problem comes back post the codes and try to run live data on it..

What we would need to see the most is long term fuel trims, maf V and baro..


The PCM really only looks at rear 02's for fuel trim purposes during failure effects management. So it is possible that not having the 02 sims in place might have made your problem worse..