P0885: TCM power-source voltage falls despite adequate system voltage

P0885 in the cited GM transmission table concerns the TCM power-relay control circuit. The documented fault criterion is a power-source reading below 8.4 volts for at least 200 milliseconds, while system voltage is above 11 volts.

The two voltage observations matter

The monitor is not simply reporting that the whole vehicle battery is low. Its enable condition expects adequate system voltage while the monitored power source is low. That distinction directs attention to the supply path represented by the TCM's measurement.

This is an interpretation of the table, not proof that the relay contacts are defective. Wiring, connections, the relay/control arrangement and measurement validity still need to be assessed with the vehicle-specific procedure.

Short interruptions can qualify

The duration is measured in milliseconds. A supply interruption capable of meeting the monitor may disappear before a technician takes a static battery reading. A normal voltage observed later therefore does not erase the original evidence.

Record the exact reporting controller, any available supply-history information and companion power-related codes before clearing the result. A qualified service test can then determine whether the fault occurs during a particular load or transition; this summary does not prescribe an improvised relay bypass.

The row gives a one-trip classification, but no universal vehicle fitment or component location. After repair, confirm that the relevant power source remains valid under the conditions that previously produced the interruption, rather than rely solely on a successful engine start.

Application limits

Document-level 2015 GM diagnostic table; per-code vehicle/engine/calibration applicability not established.

Source

15OBDG03 Transmission Diagnostics, physical PDF pages 11.