I am used to older cars and would really appreciate some kind person giving me some advice about my TJ Magna. (2000 model Solara wagon)
I have 3 questions :

1) .....I have a very large flat spot on acceleration (lowish revs, low speed, under slight load) and after reading a few threads here, decided it was most likely the mass airflow sensor. Bought a secondhand one but it hasn't fixed the problem.
In order for it to work properly, do I need to re-set the ECU by disconnecting the battery for a few minutes ?

2) .....I can see the diagnostic codes on a few websites, but cannot find the diagnostic port. Where is it on a TJ Magna ??

3) ..... As the flat spot may be caused by the oxygen sensor, I believe I need to unplug it to see if that fixes the problem. (I imagine doing this is only temporary as it will cause something else like increased fuel consumption ?). I saw that in some models the plug going into the car's wiring harness is a white wire in the passenger footwell, but I cannot see this in my car. Where is it ?

I found where the oxygen sensor plugs in (hidden behind a metal plate) and unplugged it. Problem solved.

Now just wondering whether to buy a Supercheap sensor, or something more expensive. And what will happen if I just continue to drive around with it disconnected ? Bad fuel economy ?

Preferably get a Genuine Mitsubishi O2 sensor about $140-150, but if that proves too expensive I'm pretty sure Bosch does a decent one for less money.
There are crappy generic brands that are compatible, I'd give those a miss.

I'm pretty sure a clapped out/missing 02 sensor would cause the the ECU to go in default mode which would make the engine run rich.
Not bad for the engine perse' as it will protect from pre-ignition & pistons from burning out but you'll soot up your spark plugs fairly quickly, fuel ecomomy will suffer & your cat converter won't like it much either.

I found where the oxygen sensor plugs in (hidden behind a metal plate) and unplugged it. Problem solved.

Now just wondering whether to buy a Supercheap sensor, or something more expensive. And what will happen if I just continue to drive around with it disconnected ? Bad fuel economy ?

I fitted a genuine sensor to my sedan almost 2 years ago, and a $60 PEC sensor off eBay to my wagon in July, both work fine so far. See how much Mal can do one for you.
Unplugging your sensor will have the ECU revert to a default fuel map, but your symptoms of missing and flatspotting will eventually return after a couple weeks once it relearns/adapts. Unplugging it should really be done only as a diagnosis measure in the absence of proper diagnosis equipment, not as a permanent fix.

I fitted a genuine one to my sedan, and a PEC one off eBay to my wagon, both work fine.
Unplugging your sensor will have the ECU revert to a default fuel map, but your symptoms of missing and flatspotting will eventually return after a couple weeks once it relearns/adapts. Unplugging it should really be done only as a diagnosis measure in the absence of proper diagnosis equipment, not as a permanent fix.

Well without the o2 sensor it doesn't have anything to relearn & adapt to though, so will continue to run how it is until a new sensor is hooked up. I don't see how the flat spot can relearn itself in the absence of an o2 sensor. The car will be in a constant state of open loop until the sensor is replaced, not making any changes to any mixtures as it has nothing to go by.

It will have average fuel economy though as it will be running on that super safe rich factory mix.

Well without the o2 sensor it doesn't have anything to relearn & adapt to though, so will continue to run how it is until a new sensor is hooked up.

That wasn't my experience with the wagon, but other factors may have attributed to the issues mine was facing. Your explanation make more sense.

I've ordered an oxygen sensor from Bursons. They sell NGK (same company that makes the spark plugs, so should be good)
for $ 187 or Walker (Mitsubishi) for $ 227. I ordered an NGK one, because really, a company that has made spark plugs for so long will be making a pretty good quality sensor.

To get to the oxygen sensor where it plugs into the wiring loom, I pulled back the carpet next to the console, in the passenger-side footwell. There was only one plastic clip I had to take out first.
With the carpet back, there are 2 pieces of metal plate screwed onto the side of the console. Unscrewing the front one reveals the sensor plug....wrapped in a piece of dark grey foam.

I still haven't found the diagnostic port though ! Anyone know where it is in a 2000 model Solara wagon ?

Various people in Google results (which could have been American Magnas, for all I know) have said : ....next to fuse box in engine bay.
.....Under dash in passenger side.
......Under dash in drivers side.

But I can't find it !

Up under the driver's side dash mate.

http://i62.tinypic.com/14uf50w.jpg

Thanks, dude.
I replaced the oxygen sensor and fixed the problem....the car runs better than it ever has. The thing is though, I found out that the Mass Airflow Sensor appears to not be the "hot wire" type which needs cleaning. It is a "vortex type". My car is a TJ....not sure when they started using the vortex type.

The oxygen sensor was a bugger to unscrew. I put Inox on it and it started to unscrew, then got tight again.....even when I moved it back and forth between tightening and unscrewing. So I got it to a place where the thread was a bit loose and put Inox on it overnight. No problem the next day.

Didn't have a 15 mm spanner....had to remove the top from the oxygen sensor and use a socket on the hex nut at the base. No problem....I just cut in slightly with a hacksaw and then hit it sideways.....that snapped it off easily.