Vehicles equipped whit a 4 speed automatic transmission may develop a harsh 1-2 shift and/or illumination of the Check Engine Light with code P1870 stored. This can be caused by wear of the bore for the TCC isolator and regulator valves in the valve body. This condition does not commonly occur until the transmission reaches normal operating temperature. If the valve body is worn, replacement will be necessary to correct this concern.
Check Engine Light With Code P1870 on GMC Jimmy
Average mileage: 174,814 (96,000–242,000)
Drive Trains affected: 4WD, Automatic Transmission
21 people reported this problem
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hard up shift from first to second, ( after we after we drove it for long time two days in a row...)
P1870 + Hard 1-2 shift. Replaced valve body, fluid, filter & gaskets.
Have no check engine light or code but trans shifts down really hard from 2nd to 1st
CODE P 1870 - NOT YET REPAIRED