I have a January 2014 Octavia 1.6 diesel with 34000 miles on clock. Been running perfectly but suddenly the temp gauge went from 90 to overheating and brought up a red alarm on dash. Stopped asap but noticed the temp gauge dropping back to 90 so drove home with the gauge going up and down from 90 to about 120. Checked oil and water both OK and water was not that hot. Certainly not boiling and the engine didn’t appear any hotter than normal. Took it to garage and they tried diagnostics but no alarm had been recorded. Suggested it might be a sensor but couldn’t confirm. Has anyone had similar? 

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not this comment, but this topic. Is water pump

Water pump, hot cold changeover sleeve sticking.