DTC P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature): Condition

WARNING: This page is about a different car, the 2002 Suzuki Grand Vitara. However, it is still accessible from the selected car via links, so may be relevant.

DTC P0128 is set when engine coolant temperature is lower than 167°F (75°C) even though engine was running for specified time after engine start. MIL is illuminated when ECM (PCM) detects malfunction during 2 consecutive drive cycles. Possible causes are:

ECT sensor malfunction
ECT sensor circuit malfunction
Thermostat malfunction
ECM (PCM) malfunction