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Code p1870

I have a 1996 Grand prix with 165000 miles. at about 150000 or so the trans died and I had it replaced with a used one. The car was off the road for a year or so and was never used or even started. I recently put it back on the road and the first thing I did was drive it 1400 miles. There was some initial trans fluid leaking. I check the trans fluid several times a week and always look at the spot I have been parked at for signs of leaks but haven’t seen any and the fluid level looks good. Now I drive it daily (about 100 miles a day) mostly highway miles and the SES light is on all the time and the code I get is p1870 which means there is some sort of “slipage” going on in the trans. I have a reader and have cleared the code but after about 20-30 miles the light comes back on. I feel no slipage at all and it seems to shift fine, no hard/late/early shifting. I’ve been told it could be simple like 1 or both solenoids (think they are on the side of the trans) but no one can tell me for sure. Please HELP!!!

This link might be useful: If you get far enough into it, there is a description of exactly when/why P1870 is set on a slightly younger Pontiac. One caveat. P1870 is a manufacturer defined code. It means whatever GM wants it to mean and can (but probably won’t) change between models or brands.