We see all the time in cyberspace mis-information being tought to those who are new to engine management and EPA functions.

One such myth is that the rear O2s are not needed for all they do is watchdog the CATs.
Many in adding header remove the rear O2s and either blow money on SIMMs or have nothing.

Here within 1 day timeframe we did 2 testruns, each covered over 120,000 feet of driving distances and recorded by a Ease scanner.

The only difference was removing the rear O2s, weather, drive style and car was the same.

Examine the data carefully and you can see without the rear O2s the engine runs about 2.5% plus leaner then when rear O2s were installed.
You also see engine produced less HP and the injectors are forced to be on longer to manage the leaner Long Term Fuel Trims (LTFT)




Also notice in this case when rear O2s were functional that one of them were not reporting correctly ( the CATS are installed but are gutted out) and we see the impact as to fuel demand differences from left and right heads



These tests clearly show the rear O2s have a much bigger impact on how the PCM commands fuel and truely they just cannot be removed or SIMMs used without understanding the rear O2s total role on commanded AFR.

Without the rear O2's is the PCM commanding 2.5% more fuel? Does it mean that the PCM has a different AFR target? (different than stoichiometric?)

Does it mean that the fuel consumption is also 2.5% higher?

Overall the PCM was trying to go richer to make up for the leaner condtion.
I did a PCM tune and commanded the injectors on longer by 5% richer to bring the long term fuel average back to around zero percent.

So yes without the rear O2s the PCM ends up using more fuel.

Is it wrong to say that the PCM uses the rear O2 sensors to adjust/modify the front sensors signals?
Will this change the targeted air fuel ratio?