Check Engine Light With Code P1870 on GMC Sonoma

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.

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Average mileage: 147,957 (111,000–201,662)
Drive Trains affected: 2WD, 4WD, Automatic Transmission
6 model years affected: 1996, 1997, 1998, 1999, more2000, 2002
14 people reported this problem
4 people shared problem details
2002 GMC Sonoma- 4WD120,000
My transmission, shifts hard when shifting to 1st and 2nd it starts to happen when my truck gets to a certain temperature. When this happens my truck begins to over heat, the heat drops and it lowers going back and forth, also my RPM meter rocks back and fourth. I flushed out my radiator, swapped out my thermostat, it was working fine after driving it for a week my engine service soon light came on and i got it checked and it says my transmission is slipping and it gave me a code which is P1870 what do i have to do to fix this problem.
2000 GMC Sonoma- 4WD175,000
Light on tranny not locking in to passing gear
1999 GMC Sonoma- 2WD171,000
Check engine light came on and had it checked. Code P1870 shows up. Noticed right after it came on that transmission was shifting hard going from 1st to 2nd. Trans. works fine until engine warms up and I have been stop and go city driving.I had the code reset but it came back on after 190 miles then the shift from 1st to 2nd. was harder.
1998 GMC Sonoma- 2WD120,000
Check engine light on with p1870 code, (from advance auto).
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