P0171,P0421 Mitsubishi -lancer es 2.4 what could trigger both of these codes

Question

replaced engine then p0421 came up so replaced both o2 sensors reset light didnt come back on for about 200 miles then poped up now i have p0421 prestent dtc and p0421 and p0171 pending dtc

Answer 1

P0171 Fuel trim lean Bank 1

P0421 Warm up catalytst below threshold

Having a lean fuel trim will certainly set a P0421 code. check for any exhaust leak prior to the 02 sensor.

 Here is a exhaust weld that was done on a vehicle. It also set a P0101 (Mass air flow) with a slight rough idle. The exhaust weld is obvious terrible, it had exhaust leaks, and a rough idle. The exhaust leak was redone. P0171 was solved.and the rough idle and P0101 was fixed. For your vehicle check for exhaust leaks