The engine’s coolant temperature has not reached the correct level within a specified time period.
What P0128 really means
During the warm up period right after the car is started, fuel management strategy is to run the car rich until all temperature criteria are met, then allow the oxygen sensors to take over to fine tune. Emissions are much higher than ideal, so decreasing the time for warm up is essential for lowering pollution. Coolant temperature is monitored right before the thermostat by one of the coolant system sensors.
What are the causes of a P0128 code?
For proper and efficient warm up, the coolant needs to stay trapped inside the engine to absorb the heat until it reaches the right temperature. This is the thermostat’s job. A heat-sensitive spring in the thermostat moves to open up the coolant passage when it’s hot enough, so the coolant begins to circulate to the radiator then back to the engine. In most cases, when the coolant is not reaching the right temperature fast enough, the fault lies with a thermostat either partially or completely open.
What is the severity of a P0128 code?
Minor. There is no immediate danger of breaking down on the side of the road. You should have time to plan for the repair, and set an appointment with a good mechanic. While not requiring immediate attention, this code should not be ignored indefinitely. When the coolant is not at the proper temperature, the car will run richer, which will cause carbonization (think of your gas grill!) inside the engine and overtax the catalytic converter. You might wind up spending more money if you defer a fix.
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Service article written by an ASE Master Technician