Samsung AC showing a code like dF, CF or an E/C error, what do the common ones mean and which need a technician?

Good instinct to read the code before reacting, because on Samsung a lot of what shows up is the unit telling you it is working normally, not faulting. From Samsung's own guidance: dF simply means the unit is in defrost mode (normal, mostly in heating, not a fault); CF is a filter-cleaning reminder, so clean or replace the filter and reset it; and CI is the auto-clean function running, which you can turn off from the remote. None of those need a technician. What you do act on is a blinking E or C code with a number, and on Samsung the first letter is E or C depending on the model but the meaning is the same: these are genuine faults, typically a room or heat-exchanger temperature sensor, the indoor fan not turning, or a control-board (EEPROM) error, and they want a technician who can read the exact code and fix that specific part rather than guessing. So the move is: write down the precise code and whether it blinks, rule out the normal ones (dF, CF, CI), and if it is an E or C number, book a proper AC repair and maintenance visit and give them the code, it saves diagnosis time. There are similar threads here for Daikin, Gree and O General codes if yours is a different brand. Reading the code first is what stops you paying for a callout on a filter reminder.