Thread: Removing rear O2 sensor on 01-newer PCMs -- Things you should know

one more question.. I'm not sure if i remember reading this earlier in the thred..

y wont a o2 simulator work for the rear? wont it do the test and then read the rear o2 and the sim will make it look good??

Originally Posted by blown383z28

one more question.. I'm not sure if i remember reading this earlier in the thred..

y wont a o2 simulator work for the rear? wont it do the test and then read the rear o2 and the sim will make it look good??

The cat diagnostic the PCM runs tests the oxygen storage capacity of the catalytic converter. When this test is running, the PCM will command the A/F ratio rich for an "extended" period of time to see how long it takes the rear O2 sensor to report a rich condition (when it was previously reporting a lean condition). If you look at the scan from a completely bone stock car with a functioning cat, you'll notice the rear O2 sensor signal is very slow to respond to changes in AFR compared to how fast the front O2 sensor responds. And the reason why is because the design of the catalytic converter is set up so it stores oxygen when the AFR is lean and releases it when the AFR is rich.

O2 sensor sims output a continously varying rich/lean signal which used to work on older generation OBD2 cars. These worked fine before GM added the advanced form of the cat diagnostic (starting in 2001 model year). The problem is the PCM knows when it is commanding this cat diagnostic to run and if the sim isn't outputting the exact signal the PCM is looking for when it runs this test, the test will fail or will not complete.

Basically you would need a "smart" O2 sim in order to work around the cat diagnostic. You would need the sim to watch the pre-cat O2 sensor signal so it could detect when the PCM runs the cat diagnostic test so it could feed the proper signals to the PCM on the post-cat O2 sensor signal circuit to make the PCM think there is a properly working catalytic converter installed. So far I have not seen any such device.

-ryan