I have a 2000 S40 with @70,000 miles. I have done all of the routine maintenance to it including replacing the antifreeze with Volvo stuff at 60,000 miles. In the past 2 weeks, the "Check Engine" light has come on. I got the code as "P128, Coolant Temperature below regulating temperature. The guage in the dash works and indicates "normal", it is in the same spot as it has always been. The car comes up to temp at the same rate as always, and the heater works with hot air. My initial thought was stuck open thermostat, but I now think that maybe a sensor is going out. Has anyone had this "problem"? The car still operates and runs normal. Any thoughts or info would be helpful.

 

The sensors commonly go bad.

 

I also had that problem. Changed the sensor and thermostate. A month later the light and P0128 popped up again.

I tried bending the connector fingers a little where the sensor plugs in. It's been a week and so far so good.

Ed