A defective thermostat can cause the powertrain control module (PCM) to set what diagnostic trouble code (DTC)?

A defective thermostat is known to affect engine temperature regulation, which can lead to the PCM detecting that the engine is not reaching the desired operating temperature within a specific time frame. This situation typically results in the setting of diagnostic trouble code (DTC) P0128. The P0128 code specifically indicates that the engine's coolant temperature is not reaching the intended temperature, which is often caused by a stuck open thermostat or an incorrectly calibrated thermostat that fails to regulate the engine temperature adequately.

In contrast, P0300 relates to random/multiple misfires, P0117 pertains to a coolant temperature sensor input problem, and P0420 indicates a problem with the catalytic converter efficiency. These codes are associated with different system issues and therefore wouldn’t directly result from a thermostat malfunction.