My brother in law just bought a 98 Subaru Legacy a few of months ago. Shortly after he got it, he got a CEL. I lent him my code reader and it came up with P0420, which is a Cat issue. He cleared the code and it went away for a month. Last month, the same code came back up. He cleared it again and said that he was going to bring it to the shop. When I spoke to him over the weekend, I asked him if he ever got his car fixed. He said that the hasn't come back yet, so he says that he's going to wait till the code reappears. Is there any harm to that?

How many miles are on the car?

 

I had a 95 that did the same thing at about 125k, it would throw a CEL every so often but the light would go out on it's own after a day or two. The O2 sensors get tired after a while and start acting up. It could be that you just need to put in a new O2 sensor.

The car has 185K miles on it. It's weird, but he said the couple of times it happened, it happened late at night and it was drizzling. It never happened during the day, regardless of weather. Odd, but maybe it's time to check the o2 sensors. What happens if a cat dies? Does the car die too?

Everything that I have read about this code points to the after-cat o2 sensor. My RS throws that code every few days too, just not got around to replacing it yet.

Mine comes on about every 2 months. I've tried all the cheap fixes, aside from replacing the cats. I just use the trusty Actron to clear it, so I don't have that annoying little light on.

code PO420 is catalyst eff. not O2, the cats need to be replaced.

code PO420 is catalyst eff. not O2, the cats need to be replaced.

 

Hmmm... not sure I agree with that. The computer is seeing the post cat o2 sensor having too much activity. This could be because of a bad o2 sensor which is cheaper than replacing a cat and would most likely be replaced when installing a new cat anyway. if it didn't cure the code issue then maybe a new cat is in order.

I have heard the Cat argument also..funny..I have the P0420 code too..re-occurring..yet my car has fairly new Cats on it...

 

I just gave up on mine..doesnt need to have emissions again till 2011..if the light bothers me too much I will do the electrical tape fix.

I get that code from time to time in cold weather. When it is -20 out for a month ... it will come on once a week, when it is warmer, once a month or so. In summer time it won't come on at all.

I don't worry about it.

You can try a spark plug non fouler. It pulls the sensor back out of the main exhaust stream so it reads a higher oxygen content. People often use them after gutting the cat to keep the light off.

They're $2 and change at most parts stores so if it doesn't work it's not a big loss.

Just to piggyback this thread a little.....I have a pesky P0440 evaporative emmision code fault that is driving me nuts. I have soldered and shrink-wrapped a new plug on the wire going in to the charcoal filter at the back right corner on my 99 GT. I cleared the code and all was well until I went over a big bump and it came back. I have cleared it three times now and it keeps coming back on when I hit bumps. Any idea what is causing this?

PO420 code is cat-converter low efficiency.

 

I get this almost weekly on newer subes in the shop. If they are under warranty, sometimes we're changing cats.

 

There is never anything "wrong" with the converter material though... it is my speculation that the electronics are overly sensitive.

PO420 code is cat-converter low efficiency.

 

I get this almost weekly on newer subes in the shop. If they are under warranty, sometimes we're changing cats.

 

There is never anything "wrong" with the converter material though... it is my speculation that the electronics are overly sensitive.

 

There is a TSB about that on the newer subarus.

Hmmm... not sure I agree with that. The computer is seeing the post cat o2 sensor having too much activity. This could be because of a bad o2 sensor which is cheaper than replacing a cat and would most likely be replaced when installing a new cat anyway. if it didn't cure the code issue then maybe a new cat is in order.

 

 

agree or not, but the computer is saying the cat is bad, if you wanna argue that then thats fine.

If you do ignore replacing the cat you will eventually lead to a no to hard start, reasoning is it's not doing it's job so by not replacing it, you will have these issues, 02 sensors to me are a temp fix but that code seems to come back. My 04 legacy just had a recall on that issue my ecm had to be reflashed due to that code issue and replacement cost.