OK, I have a 2001 S40 with under 72,000 miles. We have flushed the coolant, change the antifreeze, and checked the coolant temp sensor with a voltmeter. The code still comes up. The volt meter changed with temperature, so I assume it's working? We have an Acutrone scanner and are monitoring the temp. Do i need to replace with a Volvo thermostat? I have a 190 or 192 in there now . What is you'll s opinion? We shut off the code and itn comes back after days of driving.

 

which code exactly are you seeing? The way to test your engine coolant temp sensor is to measure its resistance should be high when cold (over 2000 ohms) low when hot (under 200 ohms - something like 170-180 at 190F and 150 at 210F - exact number varies by model). My bet is you need a new engine coolant temp sensor and since you need to pop off the thermostat cover to access that, replace it as well with a factory OEM unit. No point in going to a different temp thermostat - that's the temp when it opens and has nothing to do with the engine run temperature - to note the fan will kick on at a specific temp - something like 210F. If the fan never kicks on, then you either have a faulty fan relay or its the fan. If the car can't maintain 190 degrees with the thermostat wide open, then you may have a radiator issue or something mechanical going on (water pump etc).

 

I'm betting t-stat. The code for the coolant temp sensor is specifically for the sensor. The other code for the t-stat, P0128, says something about the coolant below proper temp.

 

If you have a new thermostat and ECT sensor, you probably need a software update. There is a bulletin for this.