p0420 ineff catalyst code. Comes and goes on its own???

[drewbabich]

p0420 ineff catalyst code.

 

Last year my check engine light would come on, then go off on its own and go for this code

 

All winter it was off for like 13,000 miles. Now its back again. I wonder if the weather affects it.

Either way.

 

Last year the subaru dealership guessed it was like $1000 cat converter job.

 

This week when it just came back on I went to the AUTOZONE and it gave me a print out that said three possible things.

1. Spark Plugs

2. Mass airflow sensor

3. O2 sensor or Catylitic converter

 

Anyone have experience with a p0420 that comes and goes like this? What fixed your code. I'm still getting 30 mpg's on the highway so I think it runs pretty good still. (it’s a five speed non turbo)

[GEE-OTTO]

I have had this exact same problem with my 2008 Legacy and here is why it comes and goes sometimes with long intervals.

 

The ECU only reads the cats below 3/4 and above 1/4 a tank. If your cat is slightly foul it varies on a drive by drive basis. So it may be on the threshold of what is acceptable to the ECU and will be happy for a few hundred or thousand miles then one day it will set itself off again.

 

This happened to me when I got happy and changed gas grades after running 87 for a few tanks. If i stepped up to 93 it would cause the cycle to start, but after a few tanks of 87 it would stay off for a while.

 

Went to the dealer and they instantly told me it was a bad Converter and it would be replaced as long as my car was under 80K miles.

 

No sure what year/make/model Suabru you have but if you are under 80K miles let the light come on and take it to the dealer.

[GTTuner]

Clean the MAF before you do anything.

[ehsnils]

Cleaning MAF etc. may help for a while, but I suspect that the cat is on it's last leg.

 

And if you do have a bad injector the cat is getting strained more.