1999 Town & Country- Service engine soon light on

Service Engine light came on couple days later turned off, came back on today & vehicle stopped shifting into 3rd. Later this evening was driving it and power steering went out, ABS light came on, temp gauge jumping from hot then cold. HELP b4 i drive this thing off a cliff. When Service Engine light came on originally took to autozone. these are the codes given:
P0700
P1776
P0420

I have read problem could be input or output sensor which are $17 bucks each. Going to try those while I wait for someone to give me some guidance.
Mileage is 145,000
Thanks

When the tranny stays in 2nd, the controller has gone into "limp home" mode. I am guessing you have a 4 spd tranny (LED shift indicator), since the 3 spd is all hydraulic. I think all V-6 came w/ a 4 spd, but you didn't tell us what you have. Others who have had this problem found it was due to a bad ground in the wiring harness, and many have fixed it. Read this thread:
www.automotiveforums.com/vbulletin/showthread.php?t=196775

Also, when the battery gets low, everything goes crazy. Happened to me once (belt slipping, so no charging) in my 2002 3.8L. The tranny shifter showed all gears, the windshield wipers actuated, chimes were ringing. I keep a voltage indicator plugged in a cigarette lighter now most of the time so I know the alternator is charging.

Thx for response Bill.

It is an automatic transmission. And I have found the accessory belt has fallen off. Started to rain yesterday evening. But the first problem in which the transmission would not go into 3rd started before the belt fell off.

Sorry, but you are going to have to try harder if you want help. "... it is an automatic transmission" is hardly sufficient. If you have a V-6 (still didn't bother to tell us), the "accessory belt" is a single serpentine belt that drives everything. If it fell off in one piece, it was probably loose for a long time, thus not driving your alternator, thus not charging the battery, thus all kinds of electrical problems like "limp home" mode. Anyway, I am gone from this thread,

Replace the belt AND tensioner, charge up the battery,clean the battery terminals and cable ends, then start over.