Check engine code P0420 question

[Ogre762]

I've seen P0420 on 3 of my cars and all 3 times it was a rear oxygen sensor. The rear oxygen sensor is usually only in place to verify that the cat is working properly. If the sensor goes completely out (0 voltage) it will throw a low voltage or open circuit code, but if the sensor is on it's way out, it will throw a 'Catalyst system efficiency below threshold.

[bigwayne3000]

Originally Posted by Ogre762

I've seen P0420 on 3 of my cars and all 3 times it was a rear oxygen sensor. The rear oxygen sensor is usually only in place to verify that the cat is working properly. If the sensor goes completely out (0 voltage) it will throw a low voltage or open circuit code, but if the sensor is on it's way out, it will throw a 'Catalyst system efficiency below threshold.

Good to know, thanks! CEL came back on today so I'm still planning on having it checked out eventually when I do my struts/shocks.

[essess]

Originally Posted by Ogre762

I've seen P0420 on 3 of my cars and all 3 times it was a rear oxygen sensor. The rear oxygen sensor is usually only in place to verify that the cat is working properly. If the sensor goes completely out (0 voltage) it will throw a low voltage or open circuit code, but if the sensor is on it's way out, it will throw a 'Catalyst system efficiency below threshold.

Thats not true, the way the cat montor sensors(rear o2's) is like this. your front o2 sensors are constantly reading back and forth between rich and lean because the pcm is purposly trying to get an average a/f ratio. If you watched it it on a diagnostic computer it would just like constant waves, up down up down. The rear o2 sensors, since they are reading "cleaned" exhaust, will switch, but at an extremely slow rate. The fronts, at idle, swith probably once every 2 seconds, the rears maybe once every 10ish, could be longer. The way a cat efficiency code is produced is when the cat is not cleaning the exhaust, the rear o2 will "mirror" the front o2 in switching, since they are reading the same unclean exhaust now. A properly working o2 system is the only way to set a cat efficiency code. Now sometimes people have luck getting that cat nice and hot, like driving hard or in a lower gear to keep the rpms up, to help clean it out and getting it to "light off" again and start cleaning

[bigwayne3000]

Originally Posted by essess

Thats not true, the way the cat montor sensors(rear o2's) is like this. your front o2 sensors are constantly reading back and forth between rich and lean because the pcm is purposly trying to get an average a/f ratio. If you watched it it on a diagnostic computer it would just like constant waves, up down up down. The rear o2 sensors, since they are reading "cleaned" exhaust, will switch, but at an extremely slow rate. The fronts, at idle, swith probably once every 2 seconds, the rears maybe once every 10ish, could be longer. The way a cat efficiency code is produced is when the cat is not cleaning the exhaust, the rear o2 will "mirror" the front o2 in switching, since they are reading the same unclean exhaust now. A properly working o2 system is the only way to set a cat efficiency code. Now sometimes people have luck getting that cat nice and hot, like driving hard or in a lower gear to keep the rpms up, to help clean it out and getting it to "light off" again and start cleaning

So at the end of the day it's most likely the cats themselves and I'll be spending a pretty penny to fix them.

[SummitWhite1LE]

Keep an eye out on eBay factory ones pop up, also check the members for sale area, anyone that put on headers should have a stock set laying around. They are not worth much for scape so they may still have them.

[bigwayne3000]

CEL keeps coming on and off which is annoying me in trying to figure out wtf it could possibly be.