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Probably has 2, the first being a collector instead of a Y pipe, second being what you would typically expect to see, one inlet one outlet probably another small line running up to engine compartment.
Signs...check engine light signaling an emissions problem, no exhaust coming from the tailpipe, rotten egg smell coming from tailpipe, unusual overheating for the cats themselves (glowing red after driving for a while).
Not really sure this is the problem. But after replacing everything from the electrical system and fuel system, I was wondering if the cat could be plugged or an air charget temp. sensor. I have no codes. My truck tends to hesitate and sometimes die on take off and stops. It's a 5 speed.
Disconnect the exhaust from the manifolds and take a ride. See if the issue still exists. Bear in mind the truck will behave differently running open exhaust like this and obviously...it's gonna be loud.
A vacumn gauge could help. Vac would be normal 17ish, when you start, but drop, maybe down to 4 or 5 and be steady when running with a restricted exhaust