Guys, i installed a new aftermarket cat, just after driving roughly 5-8 miles i got a CEL, checking it with a scanner it is
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
Took it to the mechanic, he checked the O2 sensors reading and according to him the second o2 sensor needs replacing as its giving quite high voltage around 1.2V. Although the sensor does fluctuate a a lot meaning its not dead or anything
Car runs fine without anyproblem, only yjhe fuel efficiency has dropped last time i didnt get a reserve light even at 370 miles with a full tank , this time i got a reserce light at 340 miles only.
Does it really mean a new sensor, they are also not that cheap, just afraid that if i replace it and the CEL comes back again .
Any help appreciated

 

possibly try something like this:
http://forums.vwvortex.com/zerothread?id=1366095

 

Reset your ECU and see if the code comes back.

Your secondary O2 sensor should have no effect on your gas mileage.

Why did you replace the cat anyway? Is it actually a cat, or a straight pipe?

 

i was thinking it was a downpipe replacement [:@]

 

Ive reset the ECU so many times , it comes back. i replaced it because i was getting a noise , but that was due to heatshield, so the mechanic ciouldnt figure it out initially and he had me replaces my genuine honda cat for no reason at all No its is not a straight pipe , it is a CAT.

You mean to say that mileage is only affected by the first sensor only ? doesnt it analysethe readings of both the sensors and then tells the engine how much fuel to inject ?
If not how come the mileage has gone down right after i got that CEL ?

 

So what does that mean ? you mean to say that vwvortex doesnt apply to me then ?

 

The primary oxygen sensor tells the ECU to adjust the air/fuel mixture. The secondary sensor only tells the ECU whether or not the cat is working.

And don't base your decline in fuel mileage in your results over just one tank, especially when you keep resetting the ECU like that, it's not helping.

 

Ahaaan .... so resetting the ECU affects the mileage ? Sorry i didnt get it when you said 'just one tank' , do you mean to say that if you have reset your ecu it re-calulates the everything which affects the mileage , can you explain that a bit more if you dont mind pls.
Also i took some readings of my O2 sensor. if i upload it , can you have a look at it and see what theb problem is . What i read from different sources, O2 coltage of the 2nd sensor should flat out to around .45 after some time. Also how do you attach a file in your post ? Thanks

 

When you reset the ECU it has to re-learn and auto-calibrate things. While it's going through all that I wouldn't expect it to get perfect gas mileage.

 

Ok . Got it , thanks . You forgot to reply to my other question

Also i took some readings of my O2 sensor. if i upload it , can you have a look at it and see what theb problem is . What i read from different sources, O2 coltage of the 2nd sensor should flat out to around .45 after some time. Also how do you attach a file in your post ? Thanks