Snagged this via google.

PO1870 is a slip code for the transmission. 99% of the time the pcm picks up the torque converter clutch slipping.
The reason the torque converter is slipping is because the valve body TCC regulator valve bore is wore out. The only way U will fix this is with a repaired valve body, new or rebuilt torque converter, and most of the time if U have gone that far U might as will rebuild the trans.
Don't wast your money on trans flushes, or fluid and filter changes, those things are transmission upkeep, and are needed to up the voscosity of the ATF. U are to late.