P0669: An implausibly high substrate reading requires a valid operating context

P0669 is the TCM substrate-temperature circuit's high-reading fault in the cited GM calibration. The monitor describes an open circuit or short to power and uses an indicated substrate temperature of at least 160°C for 60 seconds. That is different from P0634's separate internal overtemperature evaluation.

The enable conditions add context

In addition to valid engine and vehicle speeds, this row requires transmission output speed of at least 200rpm for five cumulative seconds. It also lists estimated motor power loss of at least 0.4kW for 200 cumulative seconds. Merely switching the ignition on and observing a high number does not recreate all of those conditions.

The documented recovery requires the substrate reading to fall to 150°C or below for four seconds. That pass condition describes monitor behavior; it does not independently prove that the controller has cooled or that its measurement is accurate.

Distinguish indicated heat from proven heat

Preserve the substrate value, available motor-loss estimate, output speed and related temperature faults. Where other trustworthy temperature information is available, use the correct service procedure to determine whether the extreme reading is physically credible.

Do not infer a burned controller from the number alone. The source identifies the monitored signal and electrical-fault category but supplies no terminal checks or internal repair instructions. Investigation of transmission electronics in this hybrid system requires the applicable professional isolation and diagnostic 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 205.