Rough Start and P0420

We bought a 95 Legacy a few months ago to save gas, previously spending $75 every 5 days in an Expedition. Anyway it has always started a little rough in the morning when cold, if you run the engine up to 2K for a few seconds it clears right up. But about a week ago we got a CEL and AutoZone said that it is P0420, Catalyst System Efficiency Below Threshold. Would this be plugged cats or an O2 sensor? Probably cheapest to try the O2 sensor then the cats. Can I gut the cats and run it(Montana, no emissions testing), and also how do I clear the CEL and see if it comes back? Thanks a million!

 

Jason

to clear the code you can disconnect the neg batery cable for ... over nite. that's way more than you neeed to do but it works.

 

po420 could be the cats, but more likely the front o2 sensor. get one from a subaru dealer online, ~105$. (there have been some folks who used aftermarket o2s and it did not clear up the issue. when an oem o2 finally did.)

 

you can gut the cat but why do it especially if it is working ok. it is not clogged, that would cause other issues. and if it fails and causes a code you can put an anti fouler on the rear o2 and be happy. even in a state with emissions.

 

the front o2 sensor is used to adjust the fuel air ratio so you want it working, well. the rear o2 sensor just checks to see if the cat is there doing its job. it can throw a code if the cat is bad or missing but it does not effect how the car operates. so if the cat fails just put an anti- fouler on the rear o2. if it gets blocked, replace it with a straight pipe and add the anti-fouler. then just keep the car in montana.

I work at a shop, P0420 = Catalytic Converter for sure. A bad O2 sensor could also throw the P0420 code as i've seen in some cars that come here, but more cases then others the catalytic will have to be replaced.

Thanks for the input. The car also runs a little rich when started, clears up almost immediately, and it will periodically cut out while it is warming up. The colder it is outside the worse the cutting out gets. I am leaning towards a sensor, but if that doesn't fix it then I will certainly change the cat.

The new OEM front O2 sensor fixed my problem so no need to replace the cats. Thanks for the help!!

Seems like the problem was solved, but just to add to this thread I got the same code when I had a pretty large exhaust leak (read, manifold cracked 3/4 of the way around) right before the primary O2, a new manifold solved the problem. You would know for damn sure if you had that sized exhaust leak though.

I'm going through a similar issue now with my '05 legacy. Just replaced a front 02 sensor to clear code P0051. A week later, check engine light is back on and it's code P0420. I'm at about 71,000 miles, and the cats are under warranty to I think 80,000. I had the code cleared. If it comes back I'm heading straight to my dealer.

Just replaced a front 02 sensor to clear code P0051. A week later, check engine light is back on and it's code P0420.

 

did you use a subaru o2 sensor? it makes a huge difference for the front sensor, not so much for the rear.

I used a denso sensor, which is the oem for subaru.

typically the p0420 code is a front sensor problem. since it showed up after the sensor was replaced that is probably the cause.

 

there has been a HUGE discussion on another board about the difference in quality of OEM vs the manufacturer's non oem part. i don't know and wont go in to it but it is surprising how many times a subaru OEM o2 fixes the problem.

 

did the o2 come wiht the connector on it or did you have to attache it?

 

is the new connector plugged in completely?

It came with the connector. I'm pretty sure it's plugged in correctly, otherwise the original P0051 probably wouldn't have cleared. I didn't do it myself.

 

As of today, the CEL hasn't come back on. It's been 4 days. and approx 120 miles.