Installing new exhaust and O2 was not budging so I cut the wires. I'd like to plug the hole in the new exhaust but will get and install a new sensor if needed.
Do I need to tell the ms3 there is no sensor somewhere?

 

just turn off EGO correction under

Fuel Settings -> AFR/EGO Control -> Algorithm (select No Correction)

 

Wait you mean the post cat o2 sensor? This is not your wbo2 right? I don't believe you had that sensor communicating anything to your ms3 and if it did it should not have been using it for any fuel correction. The post cat o2 is there for measuring the cats effectiveness in obd2 emission strategy. Even in factory settings its not used for fuel correction. I would say your job is done.

 

Ok, fantastic. So do not change the EGO correction?

 

The front and rear O2 sensors are not used at all with the Basic MS3.

 

are you using a wideband? if so, no need to do anything as long as the ms3 is set to look at the wideband. You won't be using the stock o2's, or at least shouldn't be

 

Did you buy that 2.5" exhaust?

 

Definitive answer:

The rear O2 sensor is completely irrelevant to any Megasquirt ECU. Its sole function is to validate the performance of the catalytic converter for the purpose of setting a fault code under OBD-II in order to cause you to fail an emissions test.

(This is one of those rare occasions when I'm not exaggerating at all.)



The forward O2 sensor *may be* relevant to a Megasquirt, depending upon how you have everything wired up. Eg: most commonly a third-party wideband sensor is installed in this position, though a stock narrowband sensor *may* be used in some instances.


But, to recap, the rear O2 sensor is *never* relevant in any Megasquirt install.

 

Well that answers a future question about using the stock narrowband with my MS3