What diagnostic trouble code (DTC) indicates a thermostat malfunction?

The diagnostic trouble code indicating a thermostat malfunction is P0128. This code is specifically associated with the engine coolant temperature not reaching the proper temperature threshold within a specified amount of time after starting the engine. The thermostat plays a crucial role in regulating the engine temperature by controlling the flow of coolant. When the thermostat is stuck open or is otherwise malfunctioning, the engine may not reach its optimal operating temperature, leading to this DTC being set.

P0125 and P0126 are related to insufficient coolant temperature or coolant temperature sensor issues but do not specifically indicate a faulty thermostat. P0130 pertains to issues with the oxygen sensor circuit, which is unrelated to thermostat functionality. Thus, when diagnosing a thermostat issue, P0128 is the correct code that accurately reflects that condition.