Less power than it should/bogging motor?

Hello, it sounds like the Catalytic Converter may have failed and is restricting the exhaust. Here are some guides to help. Converters will fail when there is an overly rich air/fuel mixture, or if the engine is burning oil and it causes the Converter to overheat it will melt down, and/or break apart and clog up the exhaust. We will usually do a back pressure test at the front oxygen sensor opening with the sensor out to see if there is excessive back pressure. I will also put a borescope down the exhaust if possible and physically look at the catalytic converter to see if it's melted down, as well as take some pictures for the customer to see. But the guides below will help. Read through the low power output guide as well.

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

https://www.2carpros.com/articles/symptoms-of-a-clogged-car-exhaust-system-a-comprehensive-guide

https://www.2carpros.com/articles/engine-has-low-power-output

Hello, I apologize for the delay, did you drill a hole in the exhaust to get that back pressure test? I have the diagrams of the exhaust system, there are 2 Catalytic converters. But this 8psi reading, it this at idle? And where is the hole location, is it up near the front oxygen sensor location?
Is that converter glowing right there? The 8psi back pressure reading is way too high. 1 to 1.5psi back pressure is the max. That thing looks like its glowing red, I wouldn't drive this vehicle anymore until you can verify that cat. I have seen vehicles catch on fire because of this.
The 2nd diagram shows the 2 converters colored in orange, the green are just exhaust heat shields.
If you unbolt the back of that converter in the 1st picture I think you're going to find that chunk of the converter is melted in one solid ball and blocking off the exhaust right there.
But you need to figure out why the cat failed, if you have a scan tool, take a look at the live engine data and check the Long Term and Short-Term Fuel Trim numbers. Right now, they are going to highly skewed due to that cat.
Once you check that exhaust, and you're going to have to check the rear catalytic converter as well, if they have both failed and they are replaced. Then when the vehicle is running ok again, look to see if those Fuel Trims are positive or negative, (for example LTFT-STFT +/- 10% or more) that needs to be corrected or the new catalytic converters will fail prematurely.
Let us know what the condition is of both cats, and then we can figure out why they failed so bad.

I posted more information if you refresh this page and look at my last post. But you should just remove one of the front oxygen sensors to measure back pressure instead of drilling holes, because with the measurement you took, you're measuring after the first converter, so you need to check in front of the first converter. Just remove one of the front oxygen sensors long enough to take a quick back pressure reading. It doesn't need to be running long. With 8 PSI, that's reading back pressure in between the 2 converters, I think you'll find even higher back pressure in front of the 1st converter.
If you unbolt those bolts that the white arrow is pointing at, you should be able to see into the back of that cat, it looks like both might be melted down.
Do you own a borescope camera?

Well using an adapter that can fit the 02-sensor hole is the way I would test it, that way a back pressure test can be done without drilling holes into the exhaust. In some cases, if you work in a shop, you'll need to be able to test the back pressure of a vehicle to verify the cat is not at fault in certain situations. They sell back pressure testing kits, or you can make an adapter out of an old 02 sensor. You only need to get a quick reading, just don't use anything rubber as an adapter, I've seen someone do that and it melted, then the piece fell into the exhaust on top of the converter inside. It was not good.

Another way is using live engine data and input the numbers into a VE calculator such as the screenshot below, you just find a road where you can do a wide-open throttle run while recording the live data. It only needs to be until you hit the highest rpm and then let off the throttle. Input all the data PIDs at the highest rpm achieved, and the "EST_VE" percentage reading is the Volumetric Efficiency of the engine, or how well the engine can take in air and let out exhaust. With a clogged (restricted exhaust) the VE number will be very low. Meaning that either no air can enter the engine, or no exhaust can escape the engine.
It is a very effective and fast test to see if there is a bad cat. Below is a link to the website with the test on it.
And one other method is using an oscilloscope with a pressure transducer screwed into the spark plug hole. The scope waveform will show high pressure on the exhaust stroke of the engine. The 2nd diagram below is an example of an in-cylinder pressure transducer reading, you can see the 2 compression stroke towers on each side, and the area where there would be excessive back pressure shown if there was a restricted exhaust.

https://atgtraining.com/atg-volumetric-efficiency-calculator/

Well, you can try that one, but you already have a gauge, so really just coming up with an adapter is all you need. This is one I made out of an old oxygen sensor, and some JB Weld.
I wouldn't pay $60.00 for that kit. The gauge you have works great. And once you have an adapter out of an old 02 sensor, you'll have one for testing in the future. I build just about as much tools as I buy. As for the current vehicle, I'm sure you want to get it done and out of there. So, your 8 PSI reading already tells you that the 2nd converter has too much back pressure. That alone already shows that you need to check it, and dropping the exhaust from the back of the first converter on a Subaru is easy. You just need a 2nd person to help you lower the entire thing once it's unbolted. I worked for a Subaru dealership, and you can take the entire exhaust down pretty easy. That way you'll be able to see into the back of the front converter. I'm sure if the rear one has failed, the front one went first.
In the 2nd diagram below, you can see that your test location right now is measuring the mid-section of the exhaust's back pressure to the 2nd cat. So, if its that high at that location, it's going to be even higher in front of the 1st cat. 8 psi is extremely high, and if you revved the engine the back pressure would go even higher. Thats why the vehicle had no power, you could probably hear the back pressure hissing while driving I bet.
Something else you could do real quick is pull the front 02 out and just see if the engine revved up better, because all that pressure would have a place to escape. It would be loud, but you would be able to slam the throttle open and get a response from the engine.
I would put a bore scope down there and look at the front cat if you don't want to drop the exhaust yet. But you already have enough proof to do so. You could buy that kit too if you needed, I think there are cheaper ones though. I use a Harbor Freight Vacuum gauge, because they also go up to 10psi which is perfect for back pressure testing.

But those seals are going to melt being in the exhaust, not sure why they would put those on there, lol.

If you work on a lot of vehicles, it is worth investing in a bore scope, I use mine to look into exhausts, in cylinders for burnt valves or leaking head gaskets, there's many uses for one. But I also know tools are expensive for sure. Let us know what you find with that cat.

Can you see from the front a bit more? Being able to see through the substrate? From that picture we can at least see that its not broken up, they are showing #13 as the Rear Catalytic converter, but have no 02 sensor behind it. And that section of pipe from the Rear Oxygen sensor (which is only for the front Cat) is a pretty long stretch, will your bore scope reach that far?

Well, we need to know why the back pressure was so high, and with no oxygen sensor behind the rear cat, it looks like it's not monitored by the ECM. Are you sure that gauge you were using is accurate? They are listing it as the rear catalytic converter, although it might not be a 3-way catalyst, but it's there to finish off any HC or CO that gets by the first cat.
With no codes to go by, it's either that the back cat is bad, or the valves are way out of adjustment on this vehicle. The valves are adjustable in this engine, it looks like they are adjustable the same way a Honda is, which a locking screw on the rocker arm.