My green m s lights, red shifter light and service engine soon light came on the other day but yesterday they did not come on after a 100 mile trip. Today they came on after a short trip. They did not come on on the trip back home. The transmission was serviced 10k miles ago, the xyz switch was replaced two years ago, the battery was replace 3 months ago, and today I checked the battery and alternator and both checked out fine. The battery was at 13.4 with the engine at idle. I don't know what could be causing my lights to come on. Any suggestions will be greatly appreciated.

 

Trans computer?
I'm just spit balling here, I have no idea since you already checked the obvious.

 

It can mean that the transmission ECU is low on voltage as it is the last one to get power from the wiring harness as I understand it. I just got dont welding on my Rover disconnected the negative cable overnight. Welded hooked it back up. M&S light flashing. Turned it off and they have not come back.
Check your negative and positive battery cable. Make sure there is a good connection. Make sure the clamp on the battery post is not bottomed out, so you cannot wiggle them by hand when fully tightend down. If you can turn by hand, your battery may not be getting fully charged by your alternator. This was my problem last Winter. I needed those lead caps that go over your battery post.
This may not solve your problem but I thought I would throw it out there.

 

may i add if youve checked codes? if check engine light has a lean code or rich code and transmission ecu has a 3 or 4th gear monitoring ratio fault code might want to replace the MAF sensor these problems are very common on discos

 

The MAF sensor was replaced about 2 year ago. I am going to have to take the truck to Autozone when I have a chance to get the codes read. I am totally stumped on this one, everything so far has checked out OK. Right now my biggest worry is something major and expensive.

 

13.4 volts won't cut it, have to be between 13.6 and 14.4 volts, so deal with that first.