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my 1996 Land Rover Discovery, when I turn on the car idles high like around 2500 to 3500, then once in awhile it try to turn off while moving and then when stop it turns off, I bought a new o2 sensor thinking that was a problem but then it started again.
I'm guessing you don't have a scan gauge or other ODBII reader. That means you're in for some experimentation.
Firstly, don't reset the ECU. You'll just be masking the problem and a few hours down the road you'll have the same problems.
It is likely to be one of two things:
If you turn the aircon on do the RPMs decrease? If yes, it might be your IACV - Idle Air Control Valve.
It can be the Throttle position sensor. Take it off and see if the revs drop to 750 or so.
Finally, don't run your vehicle in its current condition unless you want it to go into permanent limp mode. That'll require yet another ECU reset. Dealers like charging for the nose for those if you can't do it yourself.