If your printer board has the bed and/or hotend reading over 290C all the time (or displays a MAXTEMP error) and you have ruled out all wiring and thermistors as the issue then this means that the IO pins on the board you have are fried. This can happen to both thermistor inputs depending on the board if one or more is shorted out/fed high voltage (over 3.3V or 5V) at any point. In severe cases the LCD may not even light or will be blank.
This can be caused by accidentally feeding 12/24V into the wire to the thermistor port on the board. Common causes of this happening is clearing filament off your hotend and shorting out the heater line to the thermistor line. This is not something that is able to be repaired on any board and will required the board to be replaced.
Another scenario is where the casing of the heaters are carrying voltage due to poor assembly which then energizes the block. In some cases this fault only occours when the heater cartridge is heated to near print temps so it can be hard to track down. Then when the screw holding the thermistor in place is over tightened (or the insulation is compromised/doesn’t cover the entire thermistor leads) it crimps the thermistor insulation and sends that voltage into the thermistor lines. You will want to check all your lines to make sure there is no continuity between any of the thermistor and heater wires. If you do not check this and replace the board the same issue will happen again.
You can verify if there was a short to the thermistor by looking at it close on the leads. The below picture was sent in to us by one of our customers that had this happen to him. Notice the burn mark on the thermistor lead where the higher voltage (12V/24V) entered the lead and shorted the board thermistor IO pin.
If the damage is severe enough you may even see visual burn/melt marks on the CPU of the board as seen below.
This type of damage is not covered under our warranty as it falls under customer damage to the product.