Engine hesitation, stalling out

Thanks so much for answering -- concerning the MAF, no, the MAF wires were not lengthened, the Fujita CAI had a direct plug in, positioned at the same point as the stock connection.

As far as I know, all ground points were re-installed, though, to be quite honest, I'm not sure what "ground points" are (I wish I was more savvy about cars, but, sadly, I don't know much).

With just the Fujita CAI installed, the engine light was not on.

When the header was installed, yes, the O2 sensor wires were lengthened, on one of the sensors.

Oddly, no one could seem to determine which O2 sensor went where. The muffler shop exchanged the sensors, twice, having claimed the wrong part was delivered, based on not having the right plug, on the sensor, to fit the available second plug-in slot on the engine.

In the end, they said that they could not figure out which was supposed to be the pre-cat versus post-cat sensor, nor could their parts store tell them which one, based on part number, went at which position. So, the shop simply ordered what they said was the "other" sensor (the one that was not the one already installed), and installed it by splicing the old plug onto the new sensor.

So, it has both sensors, one of which, according to the parts store, is a pre-cat, and one which is a post-cat. I can't seem to get anyone to tell me if they are in the right positions.

Yes, the check engine light came on not too long after that.

My main challenge with this has been the inability to find someone who knows about tuning a Mazda3. I wanted to have it tuned after the mods, because I think it's running lean, and I know the air/fuel ratio is probably off because of the increased air flow.

My frustration has been that the dealerships don't want to address issues with a modded car, and, unfortunately, there doesn't seem to be a tuner community, or even a local shop, in my area that does work on the Mazda3.

As I say, me being a total novice, I have to rely on someone else's expertise as I'm learning. Our one performance shop closed its doors, just after I sold my heavily modded '98 Eclipse and bought this Mazda3. I had no idea it would be so difficult to get help in modding this car, but, so far, I've found that parts are hard to find, and, in Seattle, shops, or even people, to advise and assist have been hard to find as well.

Again, any help is very much appreciated.

Oh, and I noted again tonight that the car warming up makes a big difference. I let it warm up at idle for about twenty minutes, before driving it, and no hesitation or choking out this time. But, drove it later after only a few minutes' warm-up time, and again could not shift in the 2200 to 2800 rpm range without the engine starting to choke out.

I have no idea why that makes a difference.

Standing by --

Turns out the less-than-savvy folks who installed the catback exhaust chopped up the main wiring harness. Once the harness was repaired, all the rest was fine and worked beautifully. There was no problem with the intake or the mass air flow sensor or the O2 sensors. I have since sold the Mazda and now have a Scion tC which is much easier to work with in terms of modifications because Scion has a much broader connection to the aftermarket parts market than Mazda.