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I have a friend with a 2002 Chevy truck 4.8 these are the codes that are in the system. He was advised that it was the O2's but it is still running bad. Esp when its cold any help would be great.
P0171
P0174
Thank you
Nick
With both codes it mean both O2's are bad, not likely.
Some easy checks would be:
A vacuum leak causes DTCs P0171 and P0174 to set at the same time. Inspect all areas of the engine for a vacuum leak. Also inspect the PCV valve for being the correct one for this application. Make sure the engine oil fill cap is in place and that it is tight. Inspect that the engine oil dip stick is fully seated. Bad gas.