2003 Chevy Avalanche check engine light

p0420 means that the bank 1 second o2 sensor is detecting a poor catalytic converter. and you should have the catalytic converter and exhaust components checked, if that is all good then replace the o2 sensor after the catalytic converter on the side of the engine that has the #1 cylinder.

p0430 is the exact same as the one before only it is on the other exhast pipe after the catalytic converter.

p0171 means system on bank one is lean- The MAF (Mass Air Flow) Sensor is dirty or faulty or there is a vaccum leak.

and the p0174 is the exact same as the one above only its the bank 2.

so if i were you i would check the MAF sensor and see if the codes come back. if so check for a vaccum leak. they could be related or the P04's and the p01's could be completely not related.

ok thanks , F.Y.I i changed my MAF to a new one . How could i narrow it down to find the problem ? thanks

well in that case you should change the front two o2 sensors as they cannot be cleaned, this should get rid of the problem.

and before doing that i forgot to mention check and see if the wiring harness for the o2 sensors are properly connected.

oh ok thanks , im going to change my o2 sensors and i hope that goes away . o btw in the mornings my engine runs very rough until it warms up then it goes away . Rough meaning RPM goes from 300 to like 900 , until it warms up .

ok then before you change the o2 sensor change your coolant temp sensor.