I replaced the stock exhaust with an off-road pipe from Summit Racing it comes as a plain y-pipe cat back Style but it's tucked up closer too the engine to prevent scraping when off road, because my truck is EFI I had to weld for O2 sensor bungs into the pipe 2 Upstream & 2 Downstream & no catalytic converter. My downstream O2 sensors read exhaust that hasn't passed through a catalytic converter despite that fact my truck runs perfect & gas mileage hasn't been an issue other than unpleasant exhaust smell I have zero problems with air to fuel mixture and performance has never been an issue.

Theoretically speaking performance should be thrown off because the catalytic converter isnt inplace to clean the exhaust between the upstream & the downstream O2 sensors, my question is why isn't my truck having issues with air to fuel ratio like everyone told me I'd experience running no cats?

The powerhouse: I've got a 2000 Dodge Ram 1500 with a 5.9 mag Hughes F1 air gap intake manifold, CIA, quad port fuel injectors, upgraded cylinder heads (reinforced valve seats), 8 mm High radio suppression spark plug wires,

 

California emissions??

Do the "non-fouler" trick. Pick up a couple of the spark plug non-foulers from your local parts store. Drill one all the way thru, thread that onto the O2 sensor, screw the non-drilled one onto that, then screw the whole thing into the sensor bung. Your truck will stop whining about the cat.

The rear sensors DO have some small influence on mixture. Your truck is likely running a bit rich, in an attempt to heat up the cat, which will never happen.

 

The sensor before the cat is what controls your air/fuel ratio, the downstream sensor's only job is to monitor the cat and set off a check engine light if something is wrong.

 

If the sensors are far enough apart, there's no issue not running a cat.

 

As HeyYou posted, use a non-fouler or for the downstream plug the holes and tie em up above the trans.

basically the downstream is looking for a different reading than what the upstream is picking up. A drop across the cat would indicate its doing its job. Based on whatever difference it sees it can institute a longterm trim adjustment in order to, as posted, heat the cat better.

the non-fouler trick or tying up out in free air shows the type of difference the downstream is looking for and keeps the upstream as the primary A/F control