I believe that P1870 is a code for slip in the tranny... I 'll dig a little more..
The poertrain control module monitors the TCC slip by calculating the diffrence between the engine speed and the transmission output shaft speed, if the PCM detects an excessive amount of slip when the TCC is engaged then DTC 1870 sets...
Action taken when the dtc sets
The PCM inhibits TCC operation
The PCM inhibits 4th gear if the transmission is in hot mode
The PCM illuminates the MIL
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