Mechanic Job in Fort Collins, Colorado

Car began throwing a few codes within the past week. At first car began to stall and would begin shaking violently while accelerating. Replaced spark plugs, and car began starting and sputtering before dying 20 seconds after starting. Replaced IAT sensor and air filter, no change in any of the codes. Cleared codes. Believe the issue is fuel injection/catalytic converter, need help diagnosing the problem.