I bought a truck with Cal plates. I live in Phx. It failed almost all emissions test. I did plugs, wires, cap, roter, air filter, pcv valve, then i look under the truck to inspect the O2 sensor to find that there was a strait pipe in place of the CAT. At this point it has been running fine and steady (just a little uneffisiant). So I take to Muffler shop to do a new cat and muffler thinking that this would make it run great, which I thought it did. It sounded great and purrrred while it idled! So I drive less than a mile from shop and the RPM needle starts flickering around. Also the engine hesitates at all speeds every time that the needle jumps. I don't believe that the muffler guy disconnected the battery when he did his welds. Could this be the problem? Did he jack up something by not disconnecting the battery? Autozone says with these models it sounds like a fuel pump. I pulled codes just to see if any, (even though check engine light is off) and NO CODES. It did also fail the pressure test at emissions, maybe this is the reason for the stuttering??? The needle and engine only started flickering after exhaust. PLEASE HELP, Nobody can figure it out...... [:@]

 

I would try disconnecting the battery to make sure it can reset the ECM. It failed the pressure test after the exhaust was repaired? Do you hear an exhaust leak?