Help me fixit

Previous owner put on Flowmaster exhaust w/o catalytic converts. Plugged in aftermarket device on cat line to “fool” the computer. Small black box (2"x1"x3/4") with RODON p-32-80 printed like in a mirror, w/ red light. Has not run well since purchase, rich w/ frequent stalls. Any ideas? '99 5.2L

Put on a proper exhaust system and undo all the stuff the prior owner did.

The engine computer is not getting the correct signals it needs from that “small black box”. Your choices are:

1: Figure out what signals the computer needs, and modify that “small black box” to deliver them.
2: Follow Joseph Meehan’s advice.

Your vehicle’s exhaust system, as currently configured, is illegal in all 50 states.

Did you know that when you bought the truck?

If so, why did you buy the truck?

I suggest you refit the exhaust system to its legal configuration, ASAP, regardless of cost.