2001 Galant service engine light on & won't stay running/ runs rough

Try this: Disconnect the ecm control (where the air filter is housed)
Let me know if that worked.

This probably happened because the battery was disconnected. If this is the case then your car’s computer chip needs to calibrate the engine’s RPM. The same thing happened to me few years ago after I replaced my spark plugs and battery, at the beginning I thought that I screw up something, so I called the Mitsubishi dealership and they told to do the following (my memory is a little rusty but I think it went this way):
1-Disconnect the battery and wait for few minutes then reconnect it.
2-turn the car on and keep it running for 15 minutes (don’t bush the gas or change gears) while its running turn on all the electrical elements in your (radio, AC, hedlights..etc).
3- after 15 minutes change the gear to D move the car forward then change the gear to R and move it backward, you’re done.
Note: I’m not sure about step 3 so if this doesn’t work, start over ,do steps 1 and 2, and change step 3:
A- Instead of moving the car just change the gear to D, R, L3, L2, L1, and then back to P, and turn the car off.
B- If this also doesn’t work, don’t do step three at all and instead just turn the car off after 15 minutes.
If none of this works, then I guess you need to call the Mitsubishi dealer guys and ask them nicely what the right procedure is. good luck.

Disconnected ecm and car stay on. if i connect it it dies. :(