after 103k miles i got my first check engine light. went to autozone and they pulled the code (p0420). after searching, ive found out this code could be a bad cat or o2 sensor. im out of warranty so im not going to the dealer. others have said to take it to a muffler shop and have them do a test on the cat or even just replace the cat with a test pipe.

im wondering if this a big deal? if i remove the battery cable, will that clear the code? is this something that could be a bigger problem down the road if not fixed right now?

 

i'd say check the o2 sensors.

 

Catalyst System Efficiency Below Threshold (Bank 1)

reset the ECM and see if it comes back...it could be as simple as an electrical glitch that caused it to misread and threw a code.

If it does come back, most likely the 02 sensor is bad.
Another could be a plugged up Cat.

if its a one time thing, after a few cycles it should go away... but i recommend you dont drive around for too long with that code on...its not a healthy code at all

 

ill try to reset the ecm first and see if it comes back

 

i reset the ecm and the check engine light has NOT come back on yet. maybe just a glitch like you said

 

big possibility as always...the car is very delicate to ground connections...slightest resistance on any sensors would cause it to throw a code.

 

thanks for the info

 

nvm

 

its been about 8 days since i cleared the code and it just came back. im going to autozone to check the code again but im pretty sure its going to be the same p0420. funny thing is, and im not even exaggerating, the CEL came on at the exact same location as it did last time lol. i was coming home from the gym again taking the same route as before. it always takes exactly 8 minutes to get home since its late at night and the roads are dead.

its either a bad o2 sensor or a bad cat. id rather start with the sensor but if that doesnt work then i just wasted 120 bucks. what about this aftermarket sensor?
http://cgi.ebay.com/ebaymotors/O2-Ox...Q5fAccessories
or
http://cgi.ebay.com/ebaymotors/Direc...37b0e6ce#payId

is there anyway to determine if its a bad cat or sensor first before i buy something i dont need?

btw, which cat is this code referring too?

 

well, im my audi, I got the same code. It ended up being the o2 sensor. One of the wires on the sensor was broken.
hopefully this helps.

 

that would be bank 1....which is the rear secondary sensor.

id try swapping the sensor with the one in the front, to see if you get a diffferent code that refers to bank 2...this would confirm 100% bad sensor.

im quiet sure it is the o2 sensor, but just to get the "could be smth else" feeling, outta the gut

 

that's a good idea about swapping the sensors. according to this picture, it looks like there are 4 sensors. 9, 10, 11 and 13. which sensor is p0420 referring too? and which one would i swap it with?



oh yea, where the hell are the cats located lol. i think one is behind the engine by the firewall (3rd cat). where is the other two? can i access them without putting the car on a lift?

 

bump..does anyone know which sensor is the rear secondary sensor? is it 9 or 13

 

13

 

are you being serious as where the cats are???

 

In this pic, you're looking at the pre-cats.

You don't NEED a lift but it would be nice. Jackstands would work too.

 

a stupid response sometimes gets better answers lol

 

couple things before you start wrenching and yanking on those O2 sensors: buy or borrow an O2 Sensor Socket...yes, they're special. And the sensor leads cannot be unplugged until you remove the entire connection terminal from the engine mounted bracket - so above, illustration 12, that connector plugs into a terminal mounted to a bracket on the frame. You have to remove the terminal from the frame bracket (total bitch) before unplugging the two leads.

this is coming from a guy that tugged on those SOBs for 15 minutes before learning the hard way.

**also, I've gotten P0420's when a weld on my pre-cat pipe failed. check for leaks/cracks if the sensor swap doesn't work.

 

you can't swap sensors don't waist your time the front wont plug in the rear socket and vice versa they are made like that on purpose so the techs don't make a mistake during service

 

eh? I'm pretty sure you could swap the both tops or both bottoms. Upper's and Lowers have different sensor leads but I'm sure you could switch and upper for an upper.

 

i changed the sensor and cleared all the codes and the check engine light has not come back on. hopefully that solved the problem.

an o2 sensor socket is a must to change these sensors. the socket was only 8 bucks. i was able to jack the front of the car up enough to get my hands in there but a lift would of been much easier.

my only advice is to screw in the the sensor first then plug in the connector so the wire doesnt get all twisted. i doubt a twisted wire would make any difference but if its to tight then it might put out the wires.