1998 Legacy GT Feels like power loss? Bad cat or O2? OBD data inside! (P0420 2 times)

My 1998 LGT 2.5 4EAT 78k Miles. It feels like something is holding the car back. No daily CEL's , idling problems, misfires, etc. Only mods to the car are Borla header, catback. Stock sized pipe.

Maintenance within the last 500 miles:

• Fuel Filter
  
• PCV Valve
  
• Air filter
  
• Seafoam in gas tank, and upper engine through Vac line
  
• Synthetic Center Diff fluid
  
• Oil
  
• Trans fluid+filter
  
• Cleaned MAF
  
• NGK V Plugs
  

Recently it has thrown the P0420 code 2 times, i cleared it each time. Appeared after driving a bit and has not came back since. The car just feels sluggish, and every once in a great while will all of a sudden have good power on takeoff. I notice this most on takeoff, to me the engine just feels like something is holding it back. Here are some O2 Sensor screen shots and logged data. The Log was from driving on the way to work for a bit. The Screenshots were taken while holding the car 2000-2500 RPM. Im thinking its a bad Cat or O2? By the charts the upstream O2 seems to be reading the data as it should. And from my understanding the dowstream O2 should have a steady voltage near .5 volts. I live in a Non emission testing state so the cat is useless to me. Let alone 2 cats (sorry tree huggers.)

If its the cats i plan on picking up a Tsudo High Flow Catted Midpipe. Also read around seeing it could be the knock sensor? From what ive read on that it seems those normally will throw a code?

(Sorry for the volume in the 2nd screenshot lol. Was more worried about the downstream as the upstream seems to be switching like its supposed too)

Any input ?

trackLog-2014-Oct-20_07-51-13.csv

Two things to check both with removing one piece. You should unbolt the cat section completely and do a visual check of the catalyst to see if the comb is clogged up with carbon. If it is clogged you will want to replace the entire section or the section that is clogged as some have the dual inline cats that are connected by a flange. IF you want a good quality replacement Hi-Flow I would look for a Random or that Tsudo like you mentioned. With the single or no cat it'll be LOUD but you seem ok with that

Once you sort that issue out I would replace the downstream (O2) sensor as well.

Two things to check both with removing one piece. You should unbolt the cat section completely and do a visual check of the catalyst to see if the comb is clogged up with carbon. If it is clogged you will want to replace the entire section or the section that is clogged as some have the dual inline cats that are connected by a flange. IF you want a good quality replacement Hi-Flow I would look for a Random or that Tsudo like you mentioned. With the single or no cat it'll be LOUD but you seem ok with that

 

Once you sort that issue out I would replace the downstream (O2) sensor as well.

This weekend i plan on pulling the cat piece to take a look. Mine does have 2 cats. I will probably stick with the high flow cat just to avoid cel's. Will update here when i find out how those cats look These seem rare to show up used. Does pretty much everyone buy them new?

If you want to rid yourself of the P0420 a Hi-Flow wont work it will merely cause the CEL to stay on constantly. I would replace it with a Walker or Bosal Stock cat section as most folks due.

If you use spark plug foulers (one drilled out with 1/2 bit) you avoid the CEL. That rear O2 is pretty useless. All its testing is the cat efficiency. Has nothing to do with air/fuel mixture. Taking the sensor out of the exhaust stream directly with the foulers tricks the O2 into seeing a normal stock cat signal

If you use spark plug foulers (one drilled out with 1/2 bit) you avoid the CEL. That rear O2 is pretty useless. All its testing is the cat efficiency. Has nothing to do with air/fuel mixture. Taking the sensor out of the exhaust stream directly with the foulers tricks the O2 into seeing a normal stock cat signal

I know all too well about that rear O2 but the fouler trick is hit or miss doesnt work on every 98-07 Impreza style layout exhaust. The sensor is affected more by temp than gas composition moving it closer would help more so than a anti-fouler IF he were to use a Hi-flow custom setup. If emissions are a non factor I would just live with the CEL as he cant use Opensource to simply turn off the P0420 DTC

Inspect the cats if they are clogged replace them with your original sensors. IF after that you get P0420 then replace the rear O2 sensor. You can have your rear O2 tested at a shop for cheap. Your upstream AFR sensor isnt important for cat efficiency readings.

Start at the cat, then sensor.

Inspect the cats if they are clogged replace them with your original sensors. IF after that you get P0420 then replace the rear O2 sensor. You can have your rear O2 tested at a shop for cheap. Your upstream AFR sensor isnt important for cat efficiency readings.

 

Start at the cat, then sensor.

My cats look good inside to me.

Whewwww

hate to bring this "old" thread up, but i ended up replacing the rear o2, car still feels very doggish and the 2-3 shift time is horridly slow imo, 2 seconds + sometimes. I have adjusted the front diff band a few times now and haven't had much difference in shifting. Small but hardly noticeable. The car has 83k miles so its nothing high milage.

Im still getting the P0420 and have 1 fouler on the rear o2, couldn't squeeze 2 foulers in there without cutting the heat shield so i guess i have to try that now for the code being thrown.

So you are still getting P0420 with a new downstream sensor? Get those foulers out of there, they arent helping the situation. If your sensors are new then the cat may be toast even if its clear inside.

New downstream yes,

I got the P0420 code and tried 1 fouler being its a common fix. Most people use 2 to get the sensor out of the stream. The code seems to come on randomly but very far and few between. Cleared the code last time and it was well over a month before it popped back up. This time while cruse was set at about 55mph.

Id rather throw the $ id spend in a new cat/tsudo pipe into my turbo swap funds lol

One other common issue is actually related to the Transmission Fluid & gunk - If the car feels sluggish (especially on take-off), it's recommended to change the fluid (cheapest you can find) - then change it again on 250 - 500 miles. Then in another 250 - 500 miles change it a 3rd time with Synthetic (Amsoil is what I use).

Note - Do not do an actual flush - just multiple short mileage fluid changes.

Seems odd - I know - but it works wonders....

One other common issue is actually related to the Transmission Fluid & gunk - If the car feels sluggish (especially on take-off), it's recommended to change the fluid (cheapest you can find) - then change it again on 250 - 500 miles. Then in another 250 - 500 miles change it a 3rd time with Synthetic (Amsoil is what I use).

 

Note - Do not do an actual flush - just multiple short mileage fluid changes.

 

Seems odd - I know - but it works wonders....

I did just change it a few thousand miles ago, but another change cant hurt, I did use synthetic last go around but that was just because i got a good deal on the fluid. If i get some extra time i may do it again

I did just change it a few thousand miles ago, but another change cant hurt, I did use synthetic last go around but that was just because i got a good deal on the fluid. If i get some extra time i may do it again

The deal here is to change it multiple times in short mileage - I changed mine only once originally myself with Synthetic too - But it really requires multiple changes in 500 miles or less.... (Use cheap fluid for the 1st two times to keep the cost down)

Maybe it'll help - certainly can't hurt;)