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Check Engine Light With Code P1870 on GMC Sierra 1500

Problem Description and Possible Solution

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.

Problem Data
RepairPal Verified
Average mileage: 149,979 (18,000–185,640)
Drive Trains affected: 2WD, 4WD, Automatic Transmission
4 model years affected: 1999, 2000, 2001, 2006
10 people reported this problem
Viewing 1 - 6 of 6 comments
hard shifting running poorly and check engine light on with code of p1870,transmission had been replacedpreviously at 136k miles. and now my 99 gmc ssierra 1500 4wd again had new transmisi put in and now i am having tons of issue now with fuel gauge acting erratic and instrument panel not working and cd player bot working at all,messenger side window has issue with motor within struggle to raise window, rear seat belts wong pull out to palos passengers to be properly secure.rear window leaks at both upper corners when sains seat is soaked. hearing popping noise within carrage. interior lights not working. horn fuse continues to blow out.
My 2001gmc serria keeps throwing the code p1870 n when it dose it wants 2shift funny
hot summer weather after 50 miles, hard shift from 1st to 2nd, only does it on hot summer days. Not fixed
Rough shift after N-O-T is reached in 1st and 2nd gear.Also it seems to want to slip out of 4th gear and fluctuate RPM's by 200 or 300 while throttle is constant.On flat ground only.,not on hills though!
hard shift from 1st to 2nd. Sometimes 2nd to 3rd. Very erratic. Comes and goes without notice. Check engine light will come on with a transmission code. Goes back off after a couple of days normal driving.
Check engine light on.engine at operating temp.hard shifts from 1to2& 3to4,,,