Hi, this is a great forum. Happy to have found you all.

I searched and read a number of topics here, but couldn't find an answer to an odd question.

 

I own a 2009 Impreza 2.5i sedan.

 

I'm seeing only a 0420 code that appeared at about 65,000 miles.

 

I monitored this with my OBD II meter while driving, after resetting the car's computer. I found that the 0420 appears immediately when the o2 HEGO monitor completes, but the cat efficiency monitor is still incomplete.

 

Could this indicate that the issue is an o2 sensor, rather than a bad cat? Wouldn't the Cat efficiency monitor need to complete in order for the 0420 to indicate a bad cat converter (or just in general that the efficiency of the converter has even been tested?).

 

Also, the engine seems to have a loss of power lately, and I read that a bad upstream o2 sensor could cause this behavior (amongst several other things.)

 

I also read that it's not uncommon for the o2 sensors to fail after 50k.

 

I'm inclined to replace both o2 sensors, but wondered if anyone had experience with seeing the 0420 after HEGO monitor completes but before the Cat efficiency monitor is complete. 

 

Thanks.

Edited February 29, 201610 yr by briel

When engine is cold, it's in a closed-loop (generic running ignoring sensors as cat isn't up to temp yet). When correct temps are reached, it exits closed loop and relies on the sensors (unless they changed this over the years, someone correct if that's the case). If it then detects an issue, it'll post a CEL.

 

 

Try reading these for advice:

https://allwheeldriveauto.com/subaru-check-engine-light-and-code-p0420-explained/

 

http://www.p0420.com/

The catalyst monitor is running at the same time as the others.

Basically as the monitor runs it ticks off a checklist of criteria that have to happen to ensure the cat system is operating properly. If any of those checks fails, it will set a code. It cannot set the monitor to Ready if there is a code present.

 

I think the cars that new have a 100k mile spark plug change interval, but it never hurts to change them early. Spark plugs can make the difference between a 420 code and no code.

Other things include a clogged air filter.

Dirty MAF sensor (if equipped)

A vacuum leak.

Or split or loose hoses for the breather/PCV system.

One way or the other, I would get a dealer to make a record of the condition. If I recall correctly, there is a warranty on emissions separate from the 5 year/60K mile standard warranty.

The sooner the better. These systems get expensive to repair on your own.

Brus that's a good point. Totally forgot about that!

Emissions warranty should be 8 years/80k miles. And that's a federal mandated warranty, so they pretty much have to honor it.