Generally when trouble code refers to bank, (that's the oxygen sensor.) some veh's have up to 4 of them, but usually when it refers to bank one, it refers to the front sensor closest to the manifold, or actually in the manifold on drivers side. which should be a heated o2 sensor, the ones after the catalytic convertor are generally not a heated sensor,
SOURCE: My 2003 3.8l chevtrolet impala produced a check
Look first to Oxygen sensors or possibly Catalytic converter is getting stopped up.
SOURCE: 2004 Chevy Impala SS 3.8L
for p0171, try: Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required. Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady). Inspect the downstream heated oxygen sensor (HO2), replace if necessary Replace the catalytic converter
In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
Inspect all vacuum and PCV hoses, replace if necessary
Check for a dirty fuel filter and proper fuel pressure
for p0420, try:
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