Trouble Code P0420 is a fairly common trouble code indicating not one particular emissions problem but yet an overall emissions efficiency fault. The description for trouble code P0420 is Catalyst System Low Efficiency. It is very important to do properly diagnosis your Pontiac G6's P0420 fault. We would expect to see additional trouble codes along with the P0420 indicating possible defective emission parts however, often while other systems are responsible for your Pontiac's Catalyst System Efficiency to perform below Threshold, the Malfunction Indicator Lamp system will not indicate a problem with those system/s and simply show the CAT is not able to lower emissions enough. The fact is in order for the catalytic converter to function efficiently it relies on proper air/fuel mixture. So any fault which could alter air/fuel mixture can potentially cause a P0420 trouble code fault.
We recommend checking your Pontiac's rear oxygen sensor first; using a professional multimeter preferably one that will show wave patterns. When doing this test while your Pontiac is warm and running in closed loop mode, the oxygen sensor waveform reading before the catalytic converter should fluctuate. The O2 sensor waveform reading AFTER the catalytic converter should be fairly steady, anywhere between .15 to .45 volts.
