I've had this or a similar fault code come up before, I Googled it and it came up with the coolant sensor so I changed it for a non genuine one, it lasted all of 2 weeks! So I bought a genuine one from Skoda about three 3 weeks ago, and now it's come back again!! The code has cleared but it must have come up again for a reason.. The coolant temp gauge never plays up, it never over heats or anything.. Not sure if it's related but sometimes if I leave it idle a while it idles slightly strange and there's some sort of vibration coming through the car.. Also again not sure if it's related, the car used to sometimes cut out, and not start etc after long journeys, I changed the crank sensor for a genuine one as that's what came up on my code reader, it hasn't not started or cut out or anything but sometimes it doesn't start great and takes a couple of cranks and idles at about 1200rpm when it does this, when it starts as it should it idles properly.

Car is an 04 Octavia VRS 1.8 20vt by the way

It points to that or the thermostat

Check the wiring to the CTS.

Never known a failing stat to cause a code unless it's on a mapped cooling system, which only a very late VRS could have.

17705

Check all the vacuum/boost piped especially the small pipes around the inlet manifold, & listen for hissing when at idle.. There's a pipe ( S pipe) under the inlet manifold which is notoroius for splitting, & is difficult to see.

Throttle Body clean is also worth doing as a dirty TB can cause 17705 too.

Lumpy idle is normally associated with vacuum/boost leaks or the ECU output of the CTS failing causing the ECU to fuel incorrectly.

If nothing comes to light you could be in for a long haul.

I've had something to do with "mapped coolant system" come up before on my code reader which pointed to the CTS when I Googled it, can't remember the code though..