P0420 = Catalyst System Efficiency Below Threshold
This means that the secondary O2 sensor detects that the catalytic converter isn't working as efficiently as it should.
This engine code can be triggered by any one of the following:
1.) The catalyst in the catalytic converter is cracked, clogged, or otherwise damaged.
2.) The secondary O2 sensor is malfunctioning.
3.) There is an exhaust leak.
4.) The computer unit is faulty.
P0171 = lean condition
Try cleaning the MAF sensor with some electical cleaner. Also check for vacuum leaks.
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Monday, December 1st, 2008 AT 1:07 AM