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: 150,457 (111,000–201,662)
Drive Trains affected: 2WD, 4WD, Automatic Transmission
6 model years affected: 1996, 1997, 1998, 1999, more2000, 2002
15 people reported this problem
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problem details
2002 GMC Sonoma - 4WD 120,000 mi,
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.
1999 GMC Sonoma - 2WD 171,000 mi,
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.
2002 GMC Sonoma - Automatic Transmission 195,000 mi,
I started having this problem with the rpm's surging up and down between 2500 and 3500. Check engine light didn't come on until 4 months of this happening. I thought I was having a fuel problem because I spent the summer of 2015 replacing the fuel pump twice and the fuel regulator. I almost bought a new injector spider. I didn't really notice any shifting problems until the check engine light came on and I pulled the p1870 code. Then i started paying attention, that it was very hard to get the truck to move from go and in reverse. It only shits hard when i give a lot of gas at one time, other than that I don't notice it, but now I think it's taking a long time to shift into 4th. If I can just only figure out why I only get 12mpg now I'll be really happy. I have a 2002 gmc sonoma crew cab 4x4.
1998 GMC Sonoma - 2WD 120,000 mi,
Check engine light on with p1870 code, (from advance auto).