My 1999 Rodeo has a CEL accompanied by the code P1870 (related to transmission).

P1870 is not one of the standard OBII codes, but is apparently a propriety code put in place by ISUZU.

Does anyone know where I can decipher this code (I called the local ISUZU dealer and they of course would not provide any info for free)?

Here is a whole thread about this code:

http://www.automotiveforums.com/vbulletin/showthread.php?t=178005&highlight=code

Hat tip to Ryan for the link from his FAQ page.

I hope this helps you with the problem.

Dale