Here are a few bulletins from GM for that trouble code if nothing can be found.
#PIP3047A: 00-01 4L80-E Harsh TCC Apply/DTC P1870 or P0741 - kw 4L80E apply automatic converter DTC hard harsh shift TCC torque transmission - (Sep 16, 2004)
The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.
2000 - 2001 light duty trucks, vans and utilities equipped with the 4L80E automatic transmission may experience a harsh TCC apply and/or a DTC P1870 or possible P0741.
Now that you have completed the service diagnostics in SI with no resolution, replace the torque converter to repair the concern.
Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.
(00 - 01 Chevrolet Suburban) and (00 - 01 GMC Yukon XL) and (00 - 01 Chevrolet Silverado) and (00 - 01 GMC Sierra) and (00 - 01 Chevrolet Express Van) and (00 - 01 GMC Savana Van)
#02-07-30-001G: Diagnostic Information on Diagnostic Trouble Code (DTC) P0894 and P1870 - (Mar 19, 2008)
This bulletin is being revised to provide information on the turbine shaft O-ring seal. Please discard Corporate Bulletin Number 02-07-30-001F (Section 07 -- Transmission/Transaxle).
Diagnostic trouble codes (DTC) P0894 and P1870, Transmission Component Slipping, may have different root causes. The following diagnostic tips are provided to assist in accurately repairing the condition.
When attempting to diagnose a DTC P0894 or P1870, it is important to also refer to the appropriate Service Information (SI) document for further possible causes of this condition.
#01-07-30-023B: Harsh 1-2 Upshift, SES, MIL, or CEL Illuminated, DTC P1870 Set (Replace Valve Body) - (Mar 22, 2002)
This bulletin is being revised to update VIN Breakpoint Date information and revise the Parts Information table. Please discard Corporate Bulletin Number 01-07-30-023A (Section 07 -- Transmission/Transaxle).
Some customers may comment on a harsh 1-2 upshift and the Check Engine Light or Service Engine Soon indicator is illuminated.
Typically, these vehicles will have been driven more than 32,000 km (20,000 mi) before this condition occurs.
The scan tool may show a DTC P1870 set as a history code.
A harsh 1-2 shift or DTC P1870, caused by wear in the control valve body, may be difficult to duplicate when the transmission temperature is below 93°C (200°F).
The condition may be due to wear in the control valve body. This wear occurs in the bore that contains the TCC isolator and regulator valves, and results in poor, or no, TCC apply.
Install a control valve body with the revised TCC regulator and isolator valves. Refer to the Parts Information table below. These valves are used in all transmissions produced after January 15, 2000 (Julian Date 0015), and all of the service parts currently available through GMSPO contain revised TCC regulator and isolator valves.
Important: If all of the following conditions are true, it is not necessary to rebuild the transmission or to replace additional transmission components beyond the control valve body.
Transmission Broadcast Code
CCD, CFD, KBD, WAD, WBD
CHD, CJD, KCD, CMD, CUD, KPD, YDD, FFD, WCD, WHD
TBD,TAD, MSD, MDD
CAD, CBD, KAD
(Includes Police Pkg.)
BFD, BWD, BKD
SHD, TLD, TBD, TAD, MSD, MDD
CCD, CFD, KBD
CJD, CHD, KCD
(L31) 5.0L (L30)
ADD, AFD, AJD, ALD, AWD, CCD, CFD, KBD, CHD, CJD, KCD
(L35) 4.3L (LF6)
TAD, MSD, CAD, CBD, KAD, WBD, SHD
(LN2) 2.2L (L43)
(Includes Police Pkg.) G Van
LHD, LSD, LUD, LPD, KXD, CPD
Parts are currently available from GMSPO.