I have a 2001 forester and the check engine light was on went to auto zone and had it scaned and needed the 02 sensor front one so I replace it. Then 2 months later check engine came on again, had it checked and it ran a coed P0420 with 3 different causes,1 Rich A/F ratio, 2 Ignition system malifuction, 3 Faulty catalytic converter. Any advice on which on and what to do?

Thanks,

Roger

PO420 is "THE" most common code. There's a multitude of things which will trigger it. I just clear it with my code reader and it usually does not come on again for many, many months.

Definately do not get sucked into replacing your cats. It will not stop the code from coming up and it will cost you a small fortune.

Edited November 6, 201015 yr by Blacky

Do you still have the old O2 sensor? What brand was the new one?

[...]needed the 02 sensor front one so I replace it. Then 2 months later check engine came on again, had it checked and it ran a coed P0420 with 3 different causes[...]

As Fairtax4me is (I think) hinting at, if the front sensor wasn't a Subaru OEM one, that might be possible cause number 4 .

As Fairtax4me is (I think) hinting at, if the front sensor wasn't a Subaru OEM one, that might be possible cause number 4 .

 

Bingo!

More than likely, the P0420 code is the result of readings from the new sensor. Search P0420 and you'll find a thousand threads about it, and plenty of info about what to do to remedy the cause.

Is it the same code that you had the first time or do you not remember?

 

Exhaust leaks, even small ones, can also cause this code.

aftermarket o2 sensors can give you problems. i would recommend finding an oem sensor. also some are heated o2 sensors and some arent. try putting in your old one and see if the new codes go away

Yes it was a Subaru brand

folks may ask if it was bought from Subaru or just a "Subaru brand" from an aftermarket store.

 

no exhaust leak?