I got some PETG filament to try on my Ender 3 Pro which I upgraded with a carbon steel nozzle and glass bed. It was printing fine but then at the same point (4 layers up) it was causing a thermal runaway. So I put it on again and watched the temperature, on the 4th layer the hotend temperature started to go down and I figured this was what was causing the thermal runaway since the temperature always seemed to be lower than it whould when the thermal runaway error happened. I managed to figure out what was causing the hotend to cool down and not be capable of staying at 230 C, it was because the part cooling fan came on at 100% after the 4th layer which cooled down the hotend too much and caused the thermal runaway error. It seems to be working fine now with the fan speed at 40%, just interested if anyone else has come across this problem. The hotend itself is supposed to be good for up to 280 C, I know the bowden tube is only up to 240 C, so why is this happenening, shouldnt the printer be designed to work fine up to its maximum temperature with cooling fans on and everything?