Any advice code PO420

[02legacygt]

So this is the code im getting
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

However it is very intermitent, and has been going on for about a year now, it will be on for a week or 2 then go off for about the same, however some times it will be on for a week or 2 then be off for a day then come right back on.

My father has a OBD2 interface for a laptop where you can read out everything in the car, sensors and crap. The first o2 sensor (a/f sensor) is reading no volts and no read out. So I figured it was bad. I changed the sensor, which let me tell you sucked to get at, and started the car it was gone. Yes i disconnected the battery.

I have driven about 200 miles and the light popped back on last night, tool it to Autozone, and it is throwing the same code again
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

WTF!!!!!!!! am I missing something?

[Okin DaVanh]

What kinda gas are you using? 87 ethanol mix?
Try Seafoaming yet? Maybe, the O2 sensor just needs to be changed.

[02legacygt]

I changed the sensor, are you saying i need to change the rear?

I thought bank 1 meant it was the front.?

[02legacygt]

Better yet, im gonna snap some pics of the read out from the OBD2 tool this weekend so everyone can see what im talking about.

[Okin DaVanh]

I changed the sensor, are you saying i need to change the rear?
I thought bank 1 meant it was the front.?

Did you perhaps get a universal 1? I too, would say it is the 1 in front. I got the same code when I bought the car in October, and have yet for it to return. Another easy way out.... Drop Chuck a PM as he helped a member clear this up recently. :wink:

[Huffer]

Pull the data from the rear O2 sensor. Both O2's run together so if the rear O2 is non functional then it won't read correctly and will make the system trigger an error.

My rule of thumb is to replace both O2s at the same time, with factory replacements. They both see the same amount of service, and you don't change your tires one at a time...

[02legacygt]

No I didnt get a universal. Its a OEM subaru right from the dealer...
What should the second o2 sensor be reading?

[Garrison]

My experience with p0420 is that if you're using uel headers, it's usually because of that. But if it's all stock, and the o2 sensors aren't to blame, then it's (a) bad catalytic converter(s).

[02legacygt]

/// fixed it, there was a loose wire in my connection to my second cat. unplugged it today to inspect and realized it was not in the connector right, fixed it, drove about 10 miles and the light turned off...

HOORAAy!!!

[Huffer]

Good stuff!

[02legacygt]

it came back with the same code... WTF!!!

i have a sneaking suspicion that there is an exhaust leak.

[chuckthefuk]

Maybe the connection is loose..

My friends forester is notorious for throwing P0420 on hard right turns.. Simple Fix ? Hard left turn :-D or clean connector and tape it to prevent it from coming loose.

Even if you have an exhaust leak it shouldn't throw that code.. I've had several types of leaks.. some where caused by bad gaskets.. some where caused by missing cat back :-D

none of these situations threw a CEL..

You can always try the spacer trick. Thread in a spacer and then install the o2 sensor.. the readings are lower .. but i doubt this is the cause.
-Chuck

[mlitz]

P04020 to me is either the rear sensor or "post cat" witch is more of a cat monitor making sure that the catalytic converter is doing its job or If you replace the sensor and the code is still coming back it is probably your converter that is going bad.

edit: oh and BANK1 means witch exhaust bank it is on... so if you had a dual exhaust with four o2 sensors two pre-cats, and two post-cats then it would tell you bank1 sensor2 ect...

[fullCircling]

if youve already replaced your front o2 sensor, chances are its the cats. yes, both.
i have a couple new magnaflows ready to get welded in, ill let you know if that takes care of my po420.
im pretty sure it will. new oem coil, plugs, wires, front o2, ground wires, and fuel filter have all been replaced. runs good, but i still get the code.

[02legacygt]

it went back off, and has been off for a while. ....

i have no idea!