This code just appeared on my 2003 S40 and it says something about a problem with the thermostat temp. Anyone know exactly what this means? Code PO128
Thanks

 

Oh yes, I know this one well. On mine, 2000 S40, P0128 says low coolant temp. It is either the thermostat or the sensor. On mine, I reset it and comes back on within a day or 2. It is looking for the proper operating temp of coolant. I have read that it also happens when you stop and restart the engine before it has reached operating temp. Not everytime, but very often. On mine, Tech has suggested changing both sensor and thermostat together, depending on age and mileage, while you are in there. I have found that if you put a resistance meter on the sensor leads when engine is cold and watch it as it warms up, the resistance will change. If it does not change, sensor is bad. Also, if meter shows either short or open circuit, sensor is bad. I checked mine and resistance changes with temperature of engine. But I am taking Tech's advice and changing both at once. Just because the sensor changed when I was watching, doesn't mean it does everytime. Also, a 2000 with 89,000 miles won't hurt it to change both anyway. Hope this helps, and again, credit to Tech.