Subject: service engine soon
Question: I know that this is not your area of expertise but maybe you can help me, and if not then maybe some suggestions. I have a 99' Mitsubishi Diamante. One day I started driving my car and my "service engine soon" light came on. Also, when i shift from (P)park down to (D)drive the (D)drive and the (3)third gear both light up and wont go into gear(its like neutral), unless I shift agian down to 2nd or 1st gear. The only way i can get (D)drive to work is if i shift all the way down and then back up to it, and even then it wont shift pass 3rd gear. Weird i know! I ended up disconnecting my battery for an hour to see if maybe it was a computer glytch and might reset itself and it worked anmd has been working fine for about 24hrs. now. Just wondering if ya have any thoughts other than take it to a dealership which i may end up doing if i cant find out anymore info. Thanx, and cant wait for a reply!
PS/ Also, no repairs have been done
Answer: Sam,
There are a few things I can think of that could cause this problem. Here are my thoughts on the possible cause:

1. Malfunction of the valve body
2. Malfunction of the Throttle Position Sensor (TPS), or improper adjustment
3. Malfunction of the Transaxle Control Module (TCM)
4. Insufficiant and/or dirty transmission fluid

Because you mentioned that you detached your battery cable and that "fixed" the problem, I would lean to believe you have an electrical problem above all else. So, the TPS or the TCM may be the problem. The automatic transmission on the Diamantes are VERY complex. You may want to check to see if your warranty still covers transmission work. I believe your car came with a 5 year/60,000 mile Drivetrain warranty from the dealership. If you haven't exceeded the 60k mile mark, you may be able to get this fixed for free. Also, you should get your transmission fluid changed every 30k-45k miles. That fluid is EXTREMELY important and should checked and changed regularly.

If this problem persists, than your only alternative may just be to have the dealership check this out. Definitly do not take your car to a regular mechanic. Some specialy transmission shops may be able to help, but just make sure they can handle advanced Japanese transmissions. But if you want to save some headaches, take you car to a Mitsubishi dealership and have it checked out. If it turns out to only be a problem with the TPS or TCM, your expenses should not be too high.
Good luck!