The engine light come on on my 1996 blazer. I was a transmission slipage fault code. I have not noticed any slipage but occasionally it shifts hard from 1st to 2nd. The light goes on and off.
The slipping code you are getting (P-1870) is coming up because the converter clutch is slipping. This is a very common problem in these. Its because the valve body is worn. To fix this problem, the valve body must be removed and the converter clutch regulator and isolator valves removed, the valve body will be reamed and sleeves installed.
Thanks transman! I was worried it might need a whole new transmission. Still sounds like an expensive repair but it is better then a new trany.