P0634: TCM substrate heat and excess supply voltage have separate trigger paths

P0634 is assigned here to excessive temperature of the transmission controller's electronic substrate. That measurement is distinct from the transmission sump temperature used by P0218 on the same page. A normal fluid-temperature display does not automatically establish that the controller's internal thermal condition is normal.

Two documented routes to failure

One route requires substrate temperature at or above 142°C for at least five seconds, after the substrate signal satisfies its validity interval. Another combines ignition voltage at or above 18V with substrate temperature at or above 50°C for at least two seconds.

This second route is why an investigation limited to fluid cooling can miss important evidence. The recorded electrical supply can change the interpretation of an otherwise moderate internal-temperature reading.

Keep the measurements separate

Preserve substrate temperature, fluid temperature and ignition voltage from the event, including duration where the diagnostic equipment supports it. Use the correct names for each parameter; an unlabeled “transmission temperature” screenshot can hide which of the two thermal measurements was actually observed.

GM lists different recovery combinations involving temperature and voltage. Those are monitor pass criteria, not a declaration that driving or continued operation is safe. The page does not specify the physical cause, a controller replacement test or a cooling repair. A qualified hybrid-transmission technician should establish signal validity and the applicable supply condition before selecting a vehicle-specific repair procedure.

Source and applicability

GM 2010 10OBDG10 Hybrid Diagnostics, only the cited controller and monitor. These are historical calibration examples; individual vehicle, engine and hardware applicability is not established. GM PDF, page 203.