P0420 Catalytic code

[tomson1355]

98 Forester, 170k. Started throwing the dreaded p0420 code. I searched the threads and found that some got away with replacing the front sensor and some had to replace the converter. So, for $100, I replaced the sensor but the code returned. Is a new cat my only hope? The local muffler shop wants $430 dollars for a new one. I found one on NAPA for $244.00. Both are direct fit. The rest of the exhaust appears to be solid.

 

Of course, I need an inspection, so I gotta do something. It seems to take a week or so before the code returns. Maybe I can squeak an inspection in in between.

 

Is there any test for the cat?

 

Any suggestions?

 

Thanks, as always.

 

Tom

Earlville, NY

[ferret]

You can always get a universal one. Like the one here REAL CHEAP !!!

 

http://cgi.ebay.com/ebaymotors/CATALYTIC-CONVERTER-SUBARU-FORESTER-1998-02_W0QQitemZ130008220521QQcmdZViewItem

 

But this would have to be taken to a local shop to have the oxygen sensor 'bung' welded on as well as the flanges. If you are good with a torch, you could also do it. Very common to do on universal converters.

 

Here of course for $140 + shipping us a direct fit:

 

http://cgi.ebay.com/ebaymotors/CATALYTIC-CONVERTER-SUBARU-FORESTER-1998_W0QQitemZ130008324181QQihZ003QQcategoryZ33629QQrdZ1QQssPageNameZWD1VQQcmdZViewItem

[nipper]

There is a second o2 sensor that is in that circuit. If that one fails it will give you the same code. Its cheaper then a cat.

 

Personally I cant agree with buying a cat off ebay. Hunt the net (keep away from easten cat's). With cats you DO get what you pay for, and with ebay sometimes there is no way to return the. By from a reputable source, especially whan your cars inspection relies on this working.

 

nipper

[Snowman]

The front oxygen sensor shouldn't be related to that code at all. It's used to monitor the air/fuel ratio in the engine. The second sensor behind the cat is the one that monitors catalyst efficiency, and hence can set that code.

 

Hook up a scan tool that can give you the voltage coming from the sensors. With the car running in closed loop, the second sensor should be hovering right around .45 volts. If it's stuck somewhere, the sensor is bad. If it's fluctuating up and down between like .3 and .7 like the front sensor, then the cat is not working and the sensor is fine.

[tomson1355]

Thanks for the input, guys. My code reader doesn't give me any live O2 sensor data. Can anybody recommend an inexpensive one that does?

 

IIRC, there are several threads here where it's said that replacing the front O2 sensor will often fix the P0420 code. I guess I shouldn't believe everything I read.

 

Tom

[tomson1355]

I found a good article on the P0420 code here:

 

http://findarticles.com/p/articles/mi_qa3828/is_200610/ai_n17196905

[OB99W]

I found a good article on the P0420 code here:

 

http://findarticles.com/p/articles/mi_qa3828/is_200610/ai_n17196905

A direct link to the PDF version of the article is http://www.motor.com/MAGAZINE/Pdf/102006_09.pdf

 

Motor Magazine articles can be found at http://www.motor.com/ ; click on "MOTOR MAGAZINE".

 

As to P0420, sometimes it's due to an O2 sensor problem, sometimes the cat itself is bad, sometimes a bad enough O2 sensor that has been ignored for a long time will cause damage to the cat and you'll need both, sometimes other engine problems kill the cat, etc. In general, if the O2 sensors have been in use for a lot of miles, it's prudent to consider changing them (especially the front one) as a first step -- if the problem goes away, great; if not, at least the new cat has a better chance of having a long life.

[nipper]

A direct link to the PDF version of the article is http://www.motor.com/MAGAZINE/Pdf/102006_09.pdf

 

Motor Magazine articles can be found at http://www.motor.com/ ; click on "MOTOR MAGAZINE".

 

As to P0420, sometimes it's due to an O2 sensor problem, sometimes the cat itself is bad, sometimes a bad enough O2 sensor that has been ignored for a long time will cause damage to the cat and you'll need both, sometimes other engine problems kill the cat, etc. In general, if the O2 sensors have been in use for a lot of miles, it's prudent to consider changing them (especially the front one) as a first step -- if the problem goes away, great; if not, at least the new cat has a better chance of having a long life.

 

 

Well with a new cat, if the sensors ahve over 100K, they should be replaced anyway.

 

Always replace the sensors first, cat second, as it may save you 300.0 bucks.

 

nipper