Powertrain, generic (extended)

P27A9 Pressure Control Solenoid C Data Incompatible

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

P27A9 concerns solenoid C. GM's controller compares the calibration it holds for this valve with the valve body actually fitted; if the block for C is absent, of the wrong device class, damaged or non-monotonic, the fault is stored at power-up and lights the MIL in that same trip (Type A in 20OBDG04 and in the 2025 table 25OBDG03A). Valve C feeds clutch C1 (CB123456) in the nine-speed, clutch C3 in the ten-speed and line pressure in the CVT; on the 2016 eight-speed it is clutch 3 (C13567). Without valid data the module cannot turn a pressure request into the right current, so clutch fill and shift timing for that element drift. The cure is loading the matching valve body data, not a new solenoid.

Likely causes

Replacement TCM fitted without solenoid C data

Valve body or solenoid assembly swapped without updating the TCM

Stored data corrupted, checksum no longer matches

Symptoms

MIL on in the first drive after the fault
Slipping or harsh application of the clutch fed by valve C
On the CVT, unstable line pressure and belt slip risk

Repair estimate

DifficultyShop_onlyWorkshop job

Parts and labour€100–400

Time0.5–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

pressure control solenoid ctcm

References: law.cornell.edu, gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com.