I bought an 08 C30 R-Design in Jan and I keep popping code P0128. Coolant not reaching temp. Heat works fine but temp rarely exceeds the 1/3 mark. Runs about 150-160°. Should I replace the thermostat and sensor or does it hurt to run it cool? Thanks

 

I'd simply replace both the ECT sensor and the thermostat. You probably need to pop the thermostat cover off to reach the senso so its a good time to do both. Since you don't know if you have a thermostat stuck open or a wonky sensor, I'd just spend the $75 and set the car right. I don' think there's a lot of risk driving for a short while but the fuel trim may be a bit off due to an incorrect temp signal. (I'd guess it'd run rich but can't say that for sure)

 

thanks for the response. Didn't think about fuel trim. I'll just replace both