I have a 3.0 litre TJ Magna, and much to my dismay, I started it up this morning, and it's decided that it doesn't want to idle, and the engine check light on the dash is coming on... It's been fine up until now... Any idea of what the problem could be?

Did you have a flat battery.or it has not enough charge.

I had exactly the same problem with my TJ 3.0L and just fixed it. It starts then suddenly dies. I went over everything in the engine bay and after squirting straight fuel into the throttle figured out the fuel/air ratio mix was obviously wrong. So first thing I looked at was the oxygen sensor which is on your exhaust just before the catalytic convertor. Make sure it is in properly and that should fix your problem unless it is dead then you may need a new one. To test it. There is a hose that goes on to your induction just before your throttle body. Get a medicine syringe and fill it with fuel and squirt fuel into your throttle body from there as someone is starting it. Pull the throttle cable slightly as well. Sometimes it took me up to 6 or 7 10mL syringe's of fuel to get it going. I was using 98 octane too. If you put the fuel in and that fixes it I would 99% say that is your problem.

Tried a new O2 sensor, still the same issue... Any other ideas?

Don't bother pouring raw fuel down the intake, not good for the catalytic converter. Anyway, the oxygen sensor isn't used when the engine starts.

Try opening the throttle a bit with your right foot next time you try to start it.
If it fires straight away, use the throttle to hold about 1,500 rpm.
Give it a minute or so to warm up.
Lift your foot slowly, and watch the tacho. If the revs drop below 750 rpm all the way down to 400, and the engine then stalls I know what the problem is.

ISC, idle speed control motor on the throttle body has died.
A temporary fix is to use the air screw on the throttle body, wind it out - anticlockwise - one turn, and try starting it. Repeat one turn at a time until you get an idle.
Not a permanent fix though, idle will be too variable.
ISC can be checked with a multimeter.

Just went through this a couple of days ago with a 1997 Lancer, had the same problem on a previous Magna too.

25 bucks at the wreckers and 2 minutes of me slapping it in, and all is well... Happy days!... Thanx, Max

Another happy customer!

Gawd I'm helpful! lol

Thanks for the feedback.