The SAME thing can occur at the connectors for the PCM, at a BCM, the IP, and bulkhead.
Bear in mind that the Delphi system uses a 5V reference regulated in the PCM as a supply for the peripheral sensors. Those PCM inputs are frequently loaded by a 2.2KΩ resistor to somewhat isolate the inputs and protect the 5V supply from overload. If you understand Ohm's Law, that means that the current flowing in a given sensor circuit is extremely low. That also means that any additional resistance will have a significant effect on the resultant signal.
In a nutshell, a corroded connection or broken down wire might show general continuity in a quick test, but a mere 30-50Ω can skew a sensor input significantly enough to place its signal out of the "normal" range to the point where the PCM does not accept it and sets a DTC. The scanner may show data for the sensor, but it would be wrong. That's generally when the mechanic picks up the wrench and starts changing parts, and the technician sets down the wrench and engages the brain.
Look again at the typical resistance range of a GM temperature sensor, and note the small differences which can result in substantial variation in interpretation by the PCM. There is a difference of less than 100Ω between normal operating temperature and overheat.
