[Phantom]

Unless you have two O2 sensors one for each bank more common on v8s and some v6s, the ECU will see lean and dump more fuel to get it to 14.7. If you have 2 O2s then you just need to send the signal from the one running all cylinders back as both sensors that will be a bit of work or you could create a fake O2 like is used for off road cars with the cat removed to eliminate the CEL.

What would be easier/better for the engine is to custom tune the engine to run leaner when the reading the idle section of the map, if the ECM allows for it.