Another P0420 Thread

[Apexwagon]

Hello,

 

I have been throwing the P0420 for a while now, I use a ScanGaugeII to pull/clear/monitor the code.

 

The car is a 2007 Legacy 2.5i with 180k miles, the code comes on randomly after either 500 miles or as low as 80 miles of driving. I commute 250mile highway trips twice a week so its mostly highway driving, but I go from 6500ft elevation to sea level each trip (and visa-versa). I notice the code comes on at/near sea level more often. I am consistently getting 29-31mpg mixed, no rough idle or difference in operation when the vehicle is in motion/running when the code is on or off.

 

I have seen many threads regarding this subject, and do not want to throw money at this car before covering the bases.

 

I used a friends infrared laser temp gauge to see the entrance/exit temps of the exhaust to/from the cat (on the piping, not the heat shields). Took the car for a long drive up a couple hills and then had it at full operating temp with a friend on the gas holding the engine at 2500rpm while on ramps/chocked for the test;

Upstream Cat- 420* pre-cat, 290* post cat

Downstream Cat- 280* pre-cat, 250* post-cat

 

Brought the car to my local mechanic and we plugged in their expensive looking diag machine and went for a drive to test voltage on both o2 sensors and there were no rapid fluctuations in voltage or anything notably malfunctioning with them, with the CEL illuminated and without. (cost me a beer a the watering hole next door)

 

My consensus with these minimal tests are that the upstream Cat is bad with the increased temp entering the cat and over 100* difference exiting the cat.

 

Anyone have mechanic experience with cats that can either prove my findings wrong, reinforce correct findings, or indicate need for further testing and provide the names of said tests? Thank you.

[relative4]

Agreed, upstream cat is bad. You've done excellent and thorough research, and your findings justify a new cat.

 

10 years is OK but not great life for a cat. Your MPG #s look great, but before replacing the cat, I'd monitor fuel trims for a drive or two to make sure you're not running rich or having occasional misfires. The swap would be a good time for new plugs just in case.

[Apexwagon]

Agreed, upstream cat is bad. You've done excellent and thorough research, and your findings justify a new cat.

 

10 years is OK but not great life for a cat. Your MPG #s look great, but before replacing the cat, I'd monitor fuel trims for a drive or two to make sure you're not running rich or having occasional misfires. The swap would be a good time for new plugs just in case.

 

Excellent, thank you and I will figure out how to obtain fuel trims on my next trip.

The car has 180k miles so I'm thinking the harshness of the previous owner (San Francisco car) had something to do with it. I have put about 50k highway on it with no major issues so far. I replaced the plugs/wires at 160k miles, so I'm hoping I didn't incorrectly gap them to run lean/rich. I guess $20 in new plugs wouldnt hurt, just the additional time involved cuts into my spring skiing

[Apexwagon]

Additionally, does anyone have a lead on a decent Cat? The vehicle is registered in Nevada, so it can be 48 state legal. Thanks.

[relative4]

180K miles is a pretty good life for a cat.

Plugs come pre-gapped these days, did you re-gap them?

Walker and Bosal make decent direct-fit cats that will neither break your bank nor last as long as OEM.

[Apexwagon]

180K miles is a pretty good life for a cat.

Plugs come pre-gapped these days, did you re-gap them?

Walker and Bosal make decent direct-fit cats that will neither break your bank nor last as long as OEM.

 

now that you mention it, I beleive when I checked the gap they were right on the money.

Thank you for the brands, will be checking/ordering!

[spacemanspif]

Your scangauge MIGHT be able to monitor fuel trims. I know I could see them on my LS swapped car and I think I can see them on my Ford Focus. Just not sure if the gauge comes ready to see the data (universal protocol) or if specific codes need to be entered in the X-gauge menus; and if the Subaru parameters are supported. The readings are +/- numbers usually no more that 5 on either side of the 0. Negative means it the computer is pulling fuel and positive means it's adding it. Check out scangauge's website to see if it reads it. Check out the torque app for your phone too, the pro version may read those data. Not sure if an access port can read cars it's not married to but if you have a friend with one it might be worth plugging in to try.

[08SpecB_DE]

I've always gone by o2 data when condemning a cat aside from the obvious ones that break apart and rattle. What is the calculated load on your motor? Do you recall what the sensors were reading? Were they close in voltage the whole time or did they bounce all over the place?

 

As far as aftermarket cats....ahh, where to begin, they're all junk. I think the longest I've seen one last is 3 years. If you plan on keeping the car go oem. If you plan on selling in the next 2 years go aftermarket, take your pick in brand as they are all pretty much the same.

[08SpecB_DE]

180K miles is a pretty good life for a cat.

Plugs come pre-gapped these days, did you re-gap them?

Walker and Bosal make decent direct-fit cats that will neither break your bank nor last as long as OEM.

 

Always check the gap on plugs unless specified on the box.

[Apexwagon]

Update*

 

Replaced the CBE that had multiple leaks as well as a rusted out separation to the muffler, 420 code gone. I’ll update after a couple thousand more miles.