Go with your GUT
 

2002 V40 286,000

OK. Its an old car. I love it. I have passed the point of no return fixing her....once more.

So here is the story....

In January I got a new flex pipe welded in. Afterwords the car had no power on the highway. Fine around the city, but dogging on the hills. downshifting on the highway.
Mileage down from 30 mpg to under 25 MPG.
Time passes....I fill her with super, power situation gets worse, an oil leak develops. I start with the easy stuff, check the air filter and remove a big mouse nest. Problem gets a little better, but doesnt go away. Finally, I bring her back to the muffler shop and they say its not their problem, must be something else. (MONROE MUFFLER) I swear at them and leave. My Gut is that the muffler/Cat is plugged. I just wanted them to disconnect it for a minute so I could diagnose. "no, thats illegal" %&%$*&

Time Passess...Now it's March. Car starts throwing a cam shaft position code, so I bring her in to a local indy shop. I let them fix the leaks (cam shaft gear seals - out of my league they also do a timining belt (it was due)). They also replace VVT and tell me to replace the PCV box. The next weekend I replace the PCV and hoses with a kit. It was sort of plugged. I guess... Not an easy job. Pain in the ***. Might have been satisfying if it solved the issue.

Car still dogging and down shifting on the highway. 55 with the pedal all the way down, shifting all over the place. Fine downhill though. I have had enough. I am not ready for another car yet. Starts throwing a 717 code which is my last straw. In a fit of frustration tonight I put the car up and cut a hole in the muffler just before the cat.
PROBLEM SOLVED. BUT LOUD CAR. Reset codes and started shopping for a new Cat.
Goal is 300,000. dont call me a quitter!. Procrastinator - definitely, Stupid maybe...but not a quitter.

I had the same problem with a Merc 280 se after changing exhaust ,no power.At tre rear you could hear a hissing sound.
Removed the rear silencer and power was back.Explanation from the shop S*** happens.
Murphy rules
loelu

Hey, i Got the same problem as you... but is there a muffler Before the cat? Isnt there just the cat, and then the resonator and then the muffler?

Some pattern-part exhaust manufacturers use some wild n wonderful silencer/muffler internals that can really strangle the engine. I bought (in times of severe hardship) a Euroflow rear box once, it hissed really loudly under power and made the car feel strained. Unlike the better boxes, this one had a capped-off inlet pipe with holes in the tube walls! Not unusual technology, but not used like that... It stopped the sound pulses coming down the pipe sure, but created extra noise instead whilst inhibiting the 'draw' through the exhaust.

You dont get silencers/mufflers before the cat, they come after.

Maybe your Lambda is filthy and needs to be cleaned?
In my petrol engine it has two. Not sure how it's with diesels if any at all.