P0420 2006 Nissan Altima

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. Oxygen sensors are also referred to as O2 sensors. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). Catalytic converters are not normally a "wear" type item, that is they are not designed to wear out and need replacement. If they have failed, it is likely due to something else that caused it to fail. This is what a P0420 means in simplified terms.

Injector cleaner will not fix this... you more than likely will need to replace the Cat. Could be $200 if you DIY with a universal cat or around $1000 if you get an OEM / Pay for labor

If the catalytic converter is needed, you could purchase a new one from A1auto.com for just over a $100, labor should run you between $135 to $175 thru a honest shop. Some charge outrageous prices just for labor! Beware, make sure you check out O2 sensors, "donut" gasket and other possible issues before embarking on the cat conv replacement. Replacing it first might still not resolve the code issue, even after a shop has done the job.

Here's what fixed my problem