P27A8: Solenoid B data fails the transmission controller’s compatibility check

P27A8 identifies invalid or incompatible characterization for pressure-control solenoid B in the reviewed GM monitor. The source maps B to GF9 torque-converter control, GR10 C2 C128910R clutch control, or CVT primary-pulley control; these are alternative applications, not interchangeable component names.

Compatibility includes the device class

One failure path occurs when the programmed characterization describes a different type of solenoid than the TCM calibration expects. That can be a meaningful distinction even if an electrical connection appears intact. The diagnostic asks whether the stored control data is appropriate for the installed device.

Missing or incomplete programming, checksum mismatch and invalid axis ordering also cause the programming-complete flag to remain false. The continuation specifies that each axis point must be greater than or equal to the preceding value.

A startup check, not an on-road pressure test

The controller evaluates this information during initialization once per normal power event. Consequently, successful operation during a brief drive does not substitute for confirming that this initialization check accepts the data.

The source ties original characterization to the valve-body assembly part number and describes corresponding TCM programming after valve-body service. Confirm assembly identity and the completed service programming record before choosing a remedy. The code alone does not prove mechanical converter or pulley damage.

Verification should show that the installed hardware has the correct accepted characterization and that a subsequent normal power-up passes the monitor. The documented response is Type A, one trip, without a universal solenoid replacement instruction.

Source and applicability

Confirm the vehicle, transmission variant and relevant module before applying this monitor description. GM diagnostic table, PDF page 1200. GM diagnostic table, PDF page 1201.