P0171 - System Too Lean - Bank 1.
P0121 - Throttle Position Sensor/Switch A Circuit Range/Performance Problem.
P0420 - Catalyst System Efficiency Below Threshold - Bank 1.
Needs to be put on a good scan tool and check the operation of the O2 sensors as well as the TPS under running conditions.
The codes suggest that you may have a vacuum leak and a faulty TPS. But without proper testing you would not be sure.
The P0420 code could be an O2 sensor issue but it could also be a failed converter.
The same testing to look at what the sensors are actually doing would show you which.
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