Nope. It'll be the stat. Its NEVER the temp sensor. (ok, almost never) To understand you need to realise the sensor has two completely seperate sensor elements inside. One feeds the dash, one feeds the ECU. The ECU fault code ("Implausible Signal") suggests that the sensor IS working, but is returning values that dont make sense. This ofcourse could be a faulty sensor, but it could also be due to the engine never reaching operating temperature. The ECU actively expects the engine to get up to 90c. If it doesnt, it knows somethings wrong. It obviously has no way of knowing if its a faulty sensor or a faulty thermostat, so it simply logs a code saying the values its receiving dont make sense. Now, we know the ECU signal is "implausible" but when you add to that the dash guage is also pegged at 60, that means two separate sensors are BOTH saying the engines not getting to 90c. At that point it becomes far less likely to be the sensors, and far more likely to be a thermostat fault.