I got an 03 awd from my friend a while back, with a blown piston. The engine was replaced (with a nice just broken in low milage) and then the transmission, because there was gear slip in third to fourth and the car wouldn't engage overdrive, and was underpowered. So the car still does'nt engage od, and is still weak. I had to replace the flex pipe a while ago so I didn't have the cash to replace anything major, but had the car checked out. First guy was a mechanical wiz with tranny's, checked the trans out and found nothing wrong. Alright, so that could have sucked, having two lemon trannys. So someone else takes the car for a ride and hooks up to the computer, and it came up as "low power to the first injector" or something of the sort. The guys dealt with mistsu's before, and suggested something's wrong with the exhaust and I need a new muffler. Before I blow $400+ on one I wanted to know if anyone had any other suggestions/ideas of what might be wrong? please and thanks you :)

Not sure what you mean by "low power to the injector" But if the car is lacking acceleration, it could be a clogged catalytic converter or muffler due to back pressure.

I've never dealt with this on a foreign car but on an american model, I hook a vacuum gauge to the intake manifold vacuum port and take a reading, then rev the engine to 2500 rpm and after a short drop, the vacuum should return to near idle readings.

Or you could unhook the cat/muffler and O2 sensor temporarily and see if performance is restored.

If the problem is still there, It could be fuel related. Check the sensors, throttle body and injectors.

Just a suggestion.

I forgot the exact reading at the time, I checked in writing and it came up as "low compression to injector 1".

I figured disconnecting the exhaust might help to check, but wasn't it technically disconnected when the flex pipe had a nice fat hole in it? Or I guess the O2 sensor throws that off, right?

I'm hoping it's something inexpensive like a sensor, but the diagnostic probably would have picked it up.

To be a bit more specific on the problem, in manual the car won't go into overdrive and is underpowered. In automatic the car will go into it but won't engage it, and from any gear automatically jumps up the 4000+rpm without any additional power

The catalytic converter was looked into already and was fine, so I'm left to assume the most likely suspect is the muffler.

Alright, the new muffler's not the fix, the car still doesn't engage overdrive.
When the car was tested the guy wrote the wrong error code, so I'll probably have to get it checked again. He remembers it being something not important like "low voltage to injector 1" or something of the sort, maybe someone can figure that out.

So, it has to be something electrical, but the question is what?