had this before with an octavia, there are 2 common failure points i know - the fan control module, held in with 2 10mm bolts under the battery tray (1J0919506 K/L) or (if fitted on your model) the thermostat housing has a 2 pin sensor that can often short and bring the fans in permanently
control module is about 100 and 'stat housing complete is approx 30 aftermarket.
Scott
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