P0171 Chevrolet - I am getting this code along with P0420

Question

My car has the check engine light on it is presenting code p0171 and p0420 at the same time. I replaced the spark plugs and the code went away for a little while and then came back on I'm not sure what the cause is I'm thinking it may be the O2 sensor before the catalytic converter but I am not sure.

Answer 1

P0171 Fuel trim system lean Bank 1

P0420 Catalyst system below threshold

Anytime the CK ENG LT comes on it affects the catalytic convertor , the purpose of the CK ENG LT is to protect the catalytic convertor.and the catalytic convertor is to protect the enviroment  from harmful emission produce by vehicles. What you have is P0171 which is a lean running condition, lean running condition are mostly caused by vacuum leaks , vacuum leaks are air that gets past the MAF (mass airflow) so if there is any vacuum leaks from ruptured vacuum hoses or disconnected vacuum hose the result is a lean running condition. you should also do a MAF cleaning and a throttle body cleaning.whenever you do a tune