PETG is commonly described as needing little or no part cooling. That advice comes from the fact that PETG's layer adhesion suffers badly when it is cooled hard, and it produces prints with excellent strength and poor overhangs.
Both problems are real. The setting that manages them is narrower than PLA's.
The working range
30 to 50% for most prints. Below about 20% overhangs sag and bridges droop; above about 60% layer bonding falls off noticeably and parts become brittle between layers.
PLA tolerates 0 to 100% and produces usable results across most of it. PETG does not, which is why a profile copied from PLA with the fan turned down rarely works well.
Fan has to be on before the overhang
Cooling that starts at the overhang layer is already late. The layer below has to be solid enough to support the first overhanging extrusion, and that means it needed cooling too.
In practice this means running the working fan speed throughout rather than trying to switch it on for specific features. Slicers that ramp fan speed by layer can do this, and configuring it correctly is more fiddly than simply leaving the fan at a moderate setting.
Temperature interacts with it
PETG prints hot — typically 230 to 250°C. At the top of that range the plastic stays molten longer and needs more cooling to hold a shape; at the bottom it sets faster and needs less.
If overhangs are poor at 50% fan, dropping the nozzle 5 to 10°C often does more than raising the fan further, and it does not cost layer strength in the same way.
The two settings have to be tuned together. Changing one without the other is why PETG profiles feel unstable.
Bridging
PETG bridges reasonably well when cooled, and badly when not. For a part with long bridges, the honest approach is to raise fan for the bridge layers specifically — most slicers expose a separate bridge fan speed, and setting it to 80 to 100% while keeping the general fan at 40% is exactly what that setting exists for.
Bridge flow and bridge speed are also separate settings and both help. Slower bridges with slightly reduced flow sag less.
Stringing is a different problem
PETG strings more than PLA and the cause is usually retraction and temperature rather than cooling. Turning the fan up to fight stringing costs layer strength and does not fix it.
The order to work through is temperature first, then retraction distance and speed, then travel speed, then coasting. Fan is not on that list.
Where the fan should not be
PETG sticks to itself and to the nozzle more than PLA does. A fan duct blowing across the nozzle rather than at the part causes temperature swings that make everything worse and can produce heat-creep clogs.
Check the duct aim with a piece of paper at nozzle height before blaming the filament.
A test that settles it
Print an overhang and bridging test at 25%, 40% and 60% fan, at your normal temperature, and snap each one. The setting that gives acceptable overhangs and still breaks with a tear rather than a clean split along a layer is the one to profile.
That is fifteen minutes of printing, it is specific to your machine and duct, and it replaces every general recommendation including this one.
Why PETG behaves this way
PETG has a wider window between melting and solidifying than PLA, and it stays tacky over a longer temperature range. That is what gives it its excellent layer bonding and its tendency to string, and it is the same property that makes an uncooled overhang sag.
Understanding that explains why the two problems trade against each other so directly, and why there is no clever setting that removes the trade. The plastic either has time to bond to the layer below or it has time to droop, and cooling is the dial between them.
Enclosures make it worse, not better
An enclosure raises ambient temperature and reduces effective cooling. PETG in an enclosure needs more fan than the same profile in open air, which is the opposite of the usual advice about enclosures.
If a profile that worked on an open printer produces sagging overhangs after the machine is enclosed, that is the reason, and raising the fan is the fix.