Code P0128 — Coolant Temperature Below Thermostat Regulating Temperature

The P0128 code means the engine is not reaching or maintaining the proper operating temperature as regulated by the thermostat. The ECM monitors coolant temperature via the ECT sensor and compares it against the expected temperature range. If the coolant takes too long to warm up or stays below the set threshold, P0128 is set. This usually points to a thermostat stuck open or a faulty coolant temperature sensor.