Sounds like your thermistor tables are wrong. You either need to figure out what thermistors you have and set your firmware appropriately, or you need to create a table (either yourself, or get it from the people who supplied it to you) and tell the firmware to use it.
Note also that there is usually a resistor on the board that works with the thermistor to figure out the temp (part of a voltage divider). Originally, this was 4.7kohms, but in some newer boards it's 1kohm (or 1K for the hot end, and 4.7k for the bed, etc). This affects which table you need to pick, so if you get it wrong your temperature readings will be out.
PS: I've seen issues where very similar part numbers are actually fairly different thermistors, due to the differences being related to things like a new variant of the thermistor which has a different Beta value. This is something to also keep in mind.
Note also that there is usually a resistor on the board that works with the thermistor to figure out the temp (part of a voltage divider). Originally, this was 4.7kohms, but in some newer boards it's 1kohm (or 1K for the hot end, and 4.7k for the bed, etc). This affects which table you need to pick, so if you get it wrong your temperature readings will be out.
PS: I've seen issues where very similar part numbers are actually fairly different thermistors, due to the differences being related to things like a new variant of the thermistor which has a different Beta value. This is something to also keep in mind.