One easy check is the charger. Chargers normally come with a green light. If it has it and won't light up, then that's your problem. If it doesn't have this indicator, then you will need a multimeter to test for voltages. This is the most common problem with laptops. If after testing your charger, your computer still doesn't work, then you'll need to take your computer apart and check some objects on your motherboard, normally what pops is the voltage regulator called MOSFET.
Found this thread for you. Another owner is having the same problem. This thread teaches you how to reset your washer's memory and hopefully the error code. Good luck http://www.applianceblog.com/mainforums/...-kenmore/12233-kenmore-he2-gives-me-error-code-f28-missing-ring.html
It is recommended if your iMac is under warranty that you contact Apple care and have them resolve this issue. If your computer is not under warranty proceed with reasonable care. While the ifixit team would like to accommodate you and provide a guide with pictures of this procedure for your specific model there are none at the moment. Your best known method to solve your problem is to loosely follow the procedure, for the 27", in the link you provided. In following that thread several people have successfully done this repair on their 24" and have stated that the repair is easier on their model. Hopefully you will find the same to be true. Credit for this procedure goes to Oggy & lefterisgr of the MacRumors Forums. This repair is not for the faint of heart, but not extremely difficult either. 1. Remove the glass, unscrew the 10 torx screws, disconnect the 4 cables that attach the LCD panel to the boards. Remove them, but be very careful, especially with the wide flat cable that's close to the middle of...
You should try reinstalling the operating system to your second hand hard disk. Maybe it still has some information (including the password) from the previous owner and that's causing you this issue, and if it does, you will require the reinstallation cds :/. Good luck
It is definitely soldered onto the main board, and you would need a fine soldering tip as it has 5 contacts really close from each other. If you are able to use your usb, then you just need to apply some solder on the usb bases, which need 4 blobs of solder on each corner (which makes it sturdy). If you wish to replace it then you would need to solder 5 contacts plus the additional 4 blobs for sturdiness.
Seems to be the MOSFET. It is possible to replace it but you might have trouble finding it. If you cannot find it then you will need to replace the motherboard. I say give it a shot and maybe you get it to work :D
It does sound like a dead capacitor. If you can't find the schematic, try looking for popped or leaked capacitors; all capacitors have their value printed onto them. Black spots on the pcb are also easy to spot to then change whatever caused such inferno ;).