CEL code P0420, on...then off and then on, and NOW off again ! ?

[SubeeTed]

Hi ALL!

I have a 2005 Legacy WGN with 157 K miles. It runs fine. Just had the oil changed, burned only 1/2 qt. in 5K miles.

Gas mileage is fine!

So....a few weeks ago I took a short cut over a rocky dirt road to avoid a bridge construction light thing. Little did I know there was a accident on that site, they sent all the traffic thru, so there saw a lot of pulling over into the rough to let traffic by !

Next day my CEL comes on, I didn't check it out, I wanted to buy a scanner. A week goes by, the CEL goes OFF, YAE !!

 

A week later it comes back on! BOO ! By that time I bought my scanner, it was dreaded code P0420, insufficient Catalyst what ever !!

Just today, another week or so later this CEL goes OFF ! I did nothing except drive my car !

What gives ?

So I'm think (OK hoping) that maybe one of my 5 O2 sensors got hit or something ? That's why it comes and goes, because from what I've seen of a real 420 code, it doesn't go away !!!

 

Can someone tell me if my car is a California emissions car?

My VIN is 4S3BP616657307620.

I might just change out the first O2 sensor, or should I change out the first 2 ???

 

HELP!!

THANKS, SubeeTed

[Rooster2]

Yes, 02 sensors have a way of setting off a check engine light, then later turning off , then back on again. I just went through this with my 99 OBW. So your on/off experience with your check engine light does not surprise me.

 

I think that a code reader should tell you more precisely what is wrong with your car. My code read out told me that my problem was only with the up stream 02 sensor, so I knew to only replace that unit. Hope this limited info helps.

 

Update go to this website to learn about your VIN number..............http://www.vindecoderz.com/EN/Subaru

Edited June 28, 2016 by Rooster2

[Fairtax4me]

On the bottom of the hood there's a sticker that says if the car is CA emisions or federal (48 state) emissions. Assuming the hood is original.

 

You need a scanner that will read live data so you can see signals from each individual O2 sensor.

[1 Lucky Texan]

any chance the exhaust system took a hit? P0420 code can def be set if exh sys leaks.

[SubeeTed]

CEL came back on today, about 24 hours going OFF! I was on a different dirt road !

I came home and hooked up my Ebay Cheapo scanner....it DIDN'T find any code !

I've used same scanner before and it DID find P0420 code !

 

Anyhow, the car is going to my buddies for other work, he has a real MAC scanner !

Maybe that'll tell a better story !

 

My car IS a California emissions car.

I look at that sticker under my hood, you have to actually read it !

It says ......... CALIFORNIA regulations....or  something like that!

 

I'll trudge on to figure something out !

I didn't add the CATAcleaner to my gas, I was going to experiment with it, my car is at that 1/4 tank, but I'll wait to we scan it !

THANKS !!! SubeeTed

[idosubaru]

You're mistaken, P0420's routinely come and go. It's very common for them to do what you're experiencing. everything you posted sounds the same as thousands of others.

 

You might notice more mention online, that is likely exacerbated by the fact that people can ignore them when intermittent and get inspections/emissions tested when the light is off. Once it's on and can't get inspected or emissions tested people are forced to do something about it.

 

If engine and gas mileage are great, P0420 is benign at least so you have no worries except state rules Nazis if applicable. I've had it on for 120,000 miles now but have ignored it for years. Great gas mileage and the engine runs great at 256,000 miles and has never left us stranded.