This is some of what the '99 VX repair manual says about the P1870 code-

The PCM monitors the difference in engine speed and transmission output speed. In D3 drive range with the TCC engaged, the engine speed should closely match transmission output speed. This DTC detects excessive TCC slip when the TCC is engaged. This is a type "B" DTC.

-Range switch malfunction could set a DTC P1870.
-A mechanical failure of the shift solenoids, TCC solenoid, or TCC PWM solenoid could set a DTC P1870.
-Internal transmission failures may set a DTC P1870.
-An intermittent or incorrect engine speed signal may set a DTC P1870.

The PCM will turn off the CEL and CTL after three consecutive ignition cycles without a failure reported.

Mark Griffin (a VX lifer)