That code means that the computer does not believe that the engine is getting up to temp. Could be that the therm is stuck open, or the therm is of the wrong operating temp. I would agree with other posters that you should perform a flush/refill and put in a new therm - I use whatever temp came stock. Probably due for that service anyway. And yea, get a GM therm - should come with a new rubber gasket. Oh, and the gauge may not be terribly accurate. I too bought a used Monte. The temp gauge read on the cold side, so I put in a new 195* therm - and the guage is still the same. A scan of the ECM shows my coolant temp at exactly 195* during operation, however.