P0842: How to Diagnose & Fix — Transmission Fluid Pressure Sensor/Switch 'A' Circuit Low

The 35R clutch wave plate inside the 6T70/6T75 six-speed transaxle cracks and fractures, allowing the 35R clutch piston to overstroke and leak apply pressure. The result is a hard flare or slip in 3rd and 5th gear and eventually a complete loss of reverse. Failures typically begin after about 25,000 miles and are extremely common on the early Lambda Acadia; GM eventually extended coverage on the wave plate to 10 years/120,000 miles. Debris from the broken plate circulates through the transmission, so a simple plate swap is often not enough on high-mileage units.

On the 2019 Ascent the mid-joint of the transmission hydraulic pressure sensor harness was built with dissimilar metals - tinplate against copper. Galvanic action forms an oxide film at that joint, and the film corrupts the pressure signal the sensor sends to the TCU. When the sensor reports a pressure HIGHER than actual, the TCU commands the valve to reduce line pressure, and the Lineartronic's drive chain loses clamping tension. Slipping chain produces noise, vibration and hesitation, and continued driving in that state leads to loss of power. Recall population was 76,842 vehicles. This is a separate campaign and a separate root cause from the later TR690 chain-guide/TCU-programming recall (21V955 / WRK-21) - that one is a programming issue at clutch engagement, this one is a corroded sensor harness.

P0700 is not a specific fault by itself — it is an umbrella/MIL-request code the ECM stores whenever the transmission control module (TCM) detects an internal transmission problem and asks the ECM to turn on the check-engine light. On the Silverado 1500 it almost always appears alongside more specific companion codes, and the underlying failure follows two dominant, well-documented patterns depending on the transmission. On trucks with the 6L80 six-speed automatic (many 5.3L and 6.2L V8 trucks, plus 2014-onward 4.3L V6 trucks; 2015-onward 6.2L trucks use the 8L90 eight-speed instead), the internal TEHCM (Transmission Electro-Hydraulic Control Module — GM's combined valve body, solenoids, pressure switches, and TCM in a single pan-mounted unit) develops internal circuit-board/solenoid faults from heat and fluid degradation, throwing P0700 together with codes like P0796, P0777, P0842, or P0973 and dropping the truck into limp mode. On older/base trucks with the 4L60E four-speed (2007-2013 4.3L/4.8L), the same P0700 is typically triggered by a failed shift solenoid (P0753/P0758) or a faulty internal mode switch. Because P0700 is a stored 'passenger' code, a scan tool that can talk to the TCM (not just a basic reader) is required to pull the specific sub-code that identifies the actual failed component.