My 1997 LeSabre (150K miles) is giving me a PO1870 code after overdrive engages, this is the only time the check engine light comes on. I can turn the light off and it comes back on again maybe 900-1200 miles later. The shifts in the first three gears are normal. Filter and fluid have been changed recently and I found nothing unusual in the pan. I used Dextron III for the refill, maybe I should have used the new Dextron VI instead. Different listings show my trans to be a 4T65E, however it has a modulator valve which would point to a 4T60E.
HotZ28 has posted in different threads the best thing to do is take it to a trans. shop and have it tested, which I intend to do after the holiday, however I have read that there is a control valve in the vb that controls the tcc and it can be removed and replaced with an updated unit after reaming the hole it went into. I was curious to see if anyone here had made that repair.
With the milage on my auto it may be better to just replace the trans, the engine still uses no oil and gets 30mpg.