Need some advice,

 

I have a Subaru Outback 2000 with over 250,xxx Miles.

Codes: P0420 and P0303

When I initially ran the reader it said the O2 sensors were bad, both front and back. I replaced those hoping it would eliminate the 420 code also but it has not. Though, the O2 sensor codes did go away.

Any advice on what would be good to check before outright replacing the Cat?

As for the 303 code I was advised that this is probably related and will go away once the 420 code is eliminated. 

 

The car doesn't run well. It lacks power and often stutters while accelerating, sometimes almost violently if the AC is on. My Battery has had some issues with a bad connection that I have to tighten frequently because someone overtightened it. 

 

Anything helps!

Regarding the battery connection. On my '99, I replaced the connector at the end of the cable, and all is well now. Seems like newer batteries have smaller posts, so the original connector at the end of the battery cable won't fit tight enough.

Car doesn't run well, suggest you do a tune up with new plugs and wires. Buy the best plugs and wires, cheapies don't work well in Subies.

A small exhaust leak between the 2 o2 sensors can cause the 0420 code. The running bad, that needs to be fixed. That is not caused by a leak.

+1 to exhaust leaks causing a p0420 - many other things can too.

 

plugs and wires - you might experiment moving the wires around if the length will allow it. see if the misfire follows the wire from cyl 3.

 

but, hard not to just replace 16 year old wires if original.

 

knock sensors crack and can cause really poor performance. One thing where cheap ebay sensors actually have a good reputation for fixing.

 

 

if you need a mechanic - post a new thread asking for a recommendation near your city.

Yeah, you should have a full tune-up. That means plugs, wires, fuel and air filters, FRESH oil change and FRESH gas. Unhook battery post so the ecu resets.

 

If the car has been running poorly for some time, it'll need 25-50 miles of heavy foot driving to help it get back in shape (O2 sensors can get covered in soot from running too rich for example) after a full tune-up.

 

If it still runs rough, you most likely have a sensor failing/failed, or maybe the cat is clogged. If still running the OEM muffler, it might have collapsed internally causing a blockage, etc. A vacuum gauge can tell you if the exhaust is obstructed amongst other engine issues. If cat is OEM, it's well past it's life expectancy. 

 

A P0420 can be: http://www.p0420.com/ read up here. 

Edited April 4, 20169 yr by Bushwick

0303 is cylinder misfire and cause of poor running most likelY.

 

I would think the 0420 has a chance of going away if you fix the misfire.

 

If you've been driving along time with it running poorly the converter May be compromised.

 

Install new NGK plugs and NGK or subaru wires

 

O2 sensor - install an O2 sensor extender, they're like $5 on eBay.

 

0420 is one big debacle of a code. It's benign and causes no drivability issues, the ECU doesn't even look at it. You can disconnect the rear sensor entirely and it changes nothing, car will run just as normal as before.

Thanks for all the suggestions!

I'm going by start by getting new battery terminals today and I will go from there. Has anyone had any complications doing this to their battery? 

http://www.thediyguy.net/2013/09/how-to-replace-battery-terminals/

Also I did get the NGK brand plugs and wires about a month ago. But they already seem to be bad. I think whatever the problem is makes them go bad almost instantly. I want to try and fix the problem before I go buy another set as to avoid wasting another pair.

 

Thanks for all the suggestions!

I'm going by start by getting new battery terminals today and I will go from there. Has anyone had any complications doing this to their battery? 

http://www.thediyguy.net/2013/09/how-to-replace-battery-terminals/

Also I did get the NGK brand plugs and wires about a month ago. But they already seem to be bad. I think whatever the problem is makes them go bad almost instantly. I want to try and fix the problem before I go buy another set as to avoid wasting another pair.

No complications when I swapped out the battery terminals. It is good to use one of those circular wire brush battery terminal post & connector cleaner tools. The tool cleans off any dirt and corrosion on the battery posts to make for a good electrical connection. Pretty hard to screw up doing this work.

 

I don't see where you need to replace the spark plugs and wires, since you just replaced them recently. They should be good for many many miles.

yeah - poor assumption was that you had original plug wires. If you got NGK or Subaru wires, they should be fine.

 

any signs of oil on the wire boots? oil inside those plug tubes can short the HV ignition.

Apply a thin coat of wheel bearing grease to all of the metal battery clamp and post.  I take all the pieces apart, use an old toothbrush.  Don't need to leave gobs of it, just get it coated.  I still have the OEM clamps on my 1987.

Update: I replaced the battery terminals and the check engine light has not come back on....YET.

How long does it usually take for the computer to reboot and throw a code?

Your battery terminals did not cause this. 

 

There is no average - those codes will trip again in a day or a year or 3 years.

 

The two codes you had often can go weeks/months before tripping again but will typically get progressively worse over time and then stay on most of the time or indefinitey.