# P0139 and P0420 - O2 and Cat replacement?



## dubnpain (Dec 5, 2006)

I received a letter from VW letting me know that I may experience a faulty catalyct converter which will trip a CEL and give an unpleasant rattle. Sure enough my CEL comes on about 2 weeks after getting the letter. I dont have VAG-COM so I used a generic reader and pulled:
P0139 - O2 Sensor Circuit, Bank1 - Sensor 2 Slow Response
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
I am just curious if I might have a bad O2 sensor or is it possible that once my cat gets replaced the O2 will start functioning again correctly? Has anyone gotton similar codes on their car and get it covered by VW? Thanks.


----------



## bearing01 (May 27, 2004)

*Re: P0139 and P0420 - O2 and Cat replacement? (dubnpain)*

It's probably the after-cat O2 sensor not working properly because it's old and slowing down.
The ECM watches that 2nd O2 sensor signal and watches if it changes signal similarly to the main before-cat O2 sensor. When both signals start to match that means the cat is not doing a good job anymore. This gives you the cat efficency P-code.
Inspect & replace the 2nd sensor. Then see if the P0420 code comes back.


----------



## bearing01 (May 27, 2004)

*Re: P0139 and P0420 - O2 and Cat replacement? (bearing01)*

Also check for any exhaust leaks.


----------



## neowave2109 (Apr 25, 2006)

*Re: P0139 and P0420 - O2 and Cat replacement? (dubnpain)*

had that problem after i changed my cat and it went away passed smog w/o problem


----------



## dubnpain (Dec 5, 2006)

*Re: P0139 and P0420 - O2 and Cat replacement? (neowave2109)*

I was told by volkwagen to replace the O2 sensor then they can replace my cat. Said the P0139 is for the sensor which is my responsibility. If that gets resolved and P0420 comes up, the cat is covered by them free of charge.
Anyone know a cheap place to get the post cat O2 sensor??


----------



## crazyzdux (Apr 19, 2006)

*Re: P0139 and P0420 - O2 and Cat replacement? (dubnpain)*

got a letter last week about a recall up to 100K and my car just triped 110K today and my cel triggered yesterday for the PO420, so just go with an aftermarket exhaust and maybe cat?


----------



## dubnpain (Dec 5, 2006)

*Re: P0139 and P0420 - O2 and Cat replacement? (crazyzdux)*

Yeah, youre going to want to replace that cat at the very least. I havent had mine replaced yet and its running like garbage. Stupid dealership being two hours away!


----------



## crazyzdux (Apr 19, 2006)

*Re: P0139 and P0420 - O2 and Cat replacement? (dubnpain)*

have to get the car inspected this month, so where can i get a good aftermarket cat maybe high flow, i figure if i have to replace i might as well replace with better parts


----------



## dubnpain (Dec 5, 2006)

*Re: P0139 and P0420 - O2 and Cat replacement? (dubnpain)*

finally got my post cat O2 replaced. old one was scorched with black soot on one side and white residue on the other. cleared the P0139 code and waiting for the CEL to come back on for cat replacement. we'll have to see..


----------



## crazyzdux (Apr 19, 2006)

*Re: P0139 and P0420 - O2 and Cat replacement? (dubnpain)*

well i dont have to worry about this anymore because the car got totaled this week, so that gets rid of that problem in a worse way


----------



## dubnpain (Dec 5, 2006)

*Re: P0139 and P0420 - O2 and Cat replacement? (crazyzdux)*

no way! how? got pics?


----------



## TRUEBELIEVER (Apr 7, 2005)

*Re: P0139 and P0420 - O2 and Cat replacement? (dubnpain)*

German autoparts has good prices and so does vw-parts-direct.com

Id just go get the cat replaced while you have the letter and its under warranty. Do that first then see what happens with the o2 sensor. Sometimes a bad cat will screw up the second o2. 

If the code comes back after the cat is replaced then at least you tried to get away without having to replace the 02 sensor.
Otherwise its like the other guy said more up top. they get old and slow and dont work as efficiantly


----------

