I've check the cable and there is no jumper wire on the 24 pins connector and also the voltages are all ok. Even went and test the PSU with a motherboard and it boots and works fine.
I've already removed the PCB (discharged the high voltage capacitors) and did a small inspection (mostly visual), remove some dust but didn't find any damaged components, at least nothing too obvious.
There is no warranty on the PSU, fix is the only option.
I had the idea that measuring the green wire to ground i would have something about 5v but I'm getting around half that (2.4v). Could be a damaged resistor or something?