One of the benefits of FI other then the ability to offroad better, is fuel economy. This does require the O2 sensor so the engine will run right during steady state conditions such as idle, and cruising. On the ORIGINAL engine the FI came on, even without the O2 sensor if everything else is functioning properly, you should get better economy with FI then with a carb.<br><br>If the engine is different then the original with headers and less exhaust back pressure, and a better less restrictive intake or any other modifications, I would be worryed about the engine running lean if there is no O2 sensor feed back. This problem is even more exagerated if it is a MAP based as opposed to MAF based FI system<br><br> I believe, but not 100% sure, that with that system you can get away with only the two before the cats. But you will need at least 2 for that compensation function to work right. <br><br>If you had catalyst in your exhaust you would need to run with a functioning O2 sensor otherwise you would ruin the catalyst.<br><br>