It's a 2014 1.6 TDI Blue Motion automatic. Been down to the village (only 6 kilometres) for a little shopping and 3 minutes into the journey back home the message that the engine was overheating flashed up. A glance at the gauge showed 120+ degrees. Turned the heater up and blower on full. Gauge started dropping rapidly; down to 90 in less than 10 seconds. Pulled over, stopped the engine and popped the bonnet. Quite clearly the engine was not overheated. The coolant reservoir was almost cool to the touch and the electric fan was not running. Started up again and the gauge read 90 which is where it normally is at and that's where it stayed until we got home. I don't know whether this is relevant but it's the first time this autumn that we've used the heater. Seems like a monitoring fault to me. Any ideas? Is this a common glitch?