I had the exact same problem, I know this is an old post but I want to leave a bit of advise to the people who might encounter this in the future.
Long story short I unwittingly shorted out the right heater cartridge and the mosfet burnt out. When I turned the machine on it started heating the right nozzle until it reached its limit and the printer turned itself off. When a mosfet on the board is letting current through you can see the corresponding led light up, most of the times when fets burn out they get stuck on the on position, so it lets the gate open for electricity to flow through and the led is always lit.
Repairing it was easy but you need to be patient and have a bit of soldering knowledge.
I got some FDS8812NZ 9fets since I couldn’t find the PSMN7R0-30YL 9 or IRF1413 7 fets readily available in my country. I researched the different models and data on them. Don’t just go buy any model, do your research and it will pay off.
You will need:
- Any of the mosfets listed above or any model close to them (N-Channel 30V 10A, SOF-8 or LFPAK are good)
- Soldering iron with fine tip 25-50W models work fine.
- Soldering wire
- Desoldering wire/mesh
- Liquid or gel Flux
- Plenty of light
- Magnifying glass
The process is simple, you need to de-solder the burnt one and solder the new one.
Watch this YouTube video to learn about Surface Mount Soldering onto circuit boards. 13
Attached are some pics I hope you find useful.