P0420 code on a 2005 Subaru Legacy Outback

I’ve had the P0420 code come up with my vehicle. Also what has been happening at least twice a week is that my check engine and cruise control lights come on and my car loses ALL acceleration. I can’t move my vehicle , so what i do is turn off my car for a few minutes and then when i turn it on again i get some acceleration in order to continue my drive. The lights stays on until the next morning. when i start the car the lights are no longer appearing until the next “episode”. Is this a cat converter issue or a sensor issue? I dont want to pay for new cat converters and i’m frightened of spending a ton of money and this issue would still be present. What would you do first?

There are recalls that may apply depending on engine size and location. The recall involves an ECM software update to prevent catalytic converter damage and possible catalytic converter replacement.

Subaru of America, Inc. (SOA) has determined that vehicles affected by this Service Program require ECM reprogramming along with catalytic converter efficiency testing and possible replacement.

0420 means the ECM isn’t seeing what it expects to see when it compares the outputs of the pre-cat O2 to the post-cat O2. It could be a sensor problem or a cat problem. Another cause is an exhaust leak that allows air into the exhaust stream that didn’t come from the combustion.

When you first start the car in the AM, the ECM doesn’t use the O2 sensors. It runs the engine in open-loop, ignoring the O2 sensors. It still runs ok, that’s the way cars used to work before the introduction of O2 sensors. It’s just that the emissions might be a little higher during open-loop phase.

The O2 sensors don’t register correct readings until they reach their operating temperature, so the ECM ignores them at first start-up in the morning. That may explain why you don’t experience this 0420 problem then.

The issue with the car losing power seems like a problem with the electrical power. Check the fused power circuits involved with the cruise control and engine areas for an intermittent problem.

After further study at a different site it seems that it it normal that the CC light comes on also when the CEL turns on.

0420 is cat efficiency code which usually means bad cat. Cats and emission issues are common for all cars and factory recalls to fix issues is also common. But, there is usually a mileage limit. 8yrs and 100k miles usually pushes you beyond the warranty assistance.

Actually, P0420 is frequently caused by a faulty O2 sensor, not a bad cat. But in this case, it sounds like faulty ECM software may be causing the driveability problems and possibly damaged the cat, to boot.

What would I do?

  1. Have the dealership install new ECM software.
  2. Drive for several days to see if the P0420 code comes back.
  3. If so, have dealership check to see if cat needs replacing.