Replace o2 sensors or not?

01Stang65 · May 19, 2020

2 years ago I installed an off road h on my 2v. Never got a code and didnt worry about it. I just got done doing a motor swap after my motor blew up and I am just paranoid about things.

If my o2 sensors were not providing error codes after the o/r install, then they must be faulty. But then I probably would have tuned them out regardless. Just wondering what you guys would do. Should I replace the O2 sensors and then clear the code or just not worry about it. Is there any harm being caused?

OneSick99GT · May 25, 2020

If its the downstream o2 sensors this effects nothing with the motor longevity wise. If its the upstreams throwing codes then yes replace them. These read the airflow lambda and makes adjustments based on the airflow mix taking place inside the motor and exiting out the exhaust header. If the oxygen sensor is bad which would be on bank 2 passenger side header then you could be running lean, too much air, or rich too much fuel . Too much fuel is better but a lean condition can lead to issues down the road. Blown piston head from too much heat or cracked piston rings. Upstream code replace the sensor immediately if its downstream don't stress it unless you want that check engine light out. I'm paranoid so I always want a clean dash in case something acts up elseware how would you know unless your dash is free of codes.

01Stang65 · May 26, 2020

Onesick99GT said:
If its the downstream o2 sensors this effects nothing with the motor longevity wise. If its the upstreams throwing codes then yes replace them. These read the airflow lambda and makes adjustments based on the airflow mix taking place inside the motor and exiting out the exhaust header. If the oxygen sensor is bad which would be on bank 2 passenger side header then you could be running lean, too much air, or rich too much fuel . Too much fuel is better but a lean condition can lead to issues down the road. Blown piston head from too much heat or cracked piston rings. Upstream code replace the sensor immediately if its downstream don't stress it unless you want that check engine light out. I'm paranoid so I always want a clean dash in case something acts up elseware how would you know unless your dash is free of codes.

My car only has 1 set of o2 sensors... I think in 99 they switched to only having 2 sensors instead of 4. 2 cats instead of 4. So I am not sure if that is considered down or upstream

OneSick99GT · May 26, 2020

It has 1 upstream oxygen sensor on bank 2 PS exhaust header. Then you have 2 downstream oxygen sensors. That's how mine is and its a 99 gt. The downstreams wont effect the pcm adjustments but if youe having issues with the upstream this provides data to the pcm to make adjustments to fuel and air which is extremely important to longevity of the motor.

01Stang65 · May 26, 2020

Onesick99GT said:
It has 1 upstream oxygen sensor on bank 2 PS exhaust header. Then you have 2 downstream oxygen sensors. That's how mine is and its a 99 gt. The downstreams wont effect the pcm adjustments but if youe having issues with the upstream this provides data to the pcm to make adjustments to fuel and air which is extremely important to longevity of the motor.

Hmmm idk my car only has 2 o2 sensors

tsemmett · Jun 2, 2020

You're definitely supposed to have 4 sensors; think about it, how could a sensor in front of the cats tell if the cats are working?

Check this article out: 99-04 X Pipe Install

Step 4 shows the connectors for the rear O2 sensors under the transmission tail (Blue/Pink connectors).
Step 7 shows the stock mid pipe (4 cats, 4 O2s).
Step 9 shows a close up of all 4 O2 sensors.

If your rear O2s were removed when the mid pipe went in, you should still have the connectors. You definitely should have been getting codes for the rear O2s though...

Replace o2 sensors or not?

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