P0668: A very low TCM substrate reading is treated as a circuit fault

For this GM TCM, P0668 identifies a substrate-temperature signal that has failed at an extremely low indicated temperature. The table associates the fault with a short-to-ground condition and sets its example threshold at −60°C or below. Preserve that source-specific relationship rather than assuming every temperature circuit has the same voltage-to-temperature behavior.

Persistence distinguishes a glitch from this monitor result

The failure must remain present for at least 60 seconds. Engine-speed and vehicle-speed values also have five-second validity requirements. A briefly implausible display is useful evidence to retain, but it is not by itself the complete documented failure condition.

Recovery is separately described as a substrate reading of at least −55°C for four seconds. The gap between the fail and pass temperatures means a slight movement away from the failure threshold may still fall short of a passing result.

Do not turn a circuit label into a replacement verdict

Record the exact substrate-temperature parameter and accompanying temperature-circuit codes. P0669 is the high-reading counterpart; P0667 compares plausible temperatures against other measurements. Those are different diagnostic questions and should not be collapsed into a generic “overheating” explanation.

This page provides no wiring terminals or resistance specification. The printed short-to-ground description does not establish where such a condition exists or whether the fault is inside the controller. Use the applicable TCM service procedure for that distinction, with hybrid-system precautions observed before any access to transmission electronics.

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.