The most probable reason that this error is generated immediately after you power on the 3D printer is that the thermistor (temperature sensor of the heated bed) is shorted. You are advised to check the cables or replace the thermistor with an appropriate alternative sensor.
If a thermistor is short-circuited, meaning it effectively has zero resistance, the behavior it reports to a microprocessor monitoring it will depend on the type of thermistor and how the temperature measurement is set up.
Generally, the type of thermistor used in 3D printing is the Negative Temperature Coefficient (NTC) thermistor. This is the most common type where resistance decreases as temperature increases, a short circuit (zero resistance) would be interpreted as an extremely high temperature. This is because the monitoring circuit (typically involving a voltage divider or a similar setup) would read the low or zero resistance as indicative of very high temperature conditions. This would trigger the bed temperature error.
E.g. for a Positive Temperature Coefficient (PTC) thermistor, where resistance increases with an increase in temperature, a short circuit would not align with the typical behavior you have described. However, since short circuits effectively bypass the thermistor, the system could be designed to recognize this as an error condition rather than a temperature reading.
In either case, sophisticated microprocessor-based systems often incorporate error handling to detect and respond to such faults. If the system recognizes the resistance reading as abnormally low (or zero), which is outside the expected range for operational temperatures, it might trigger an error or fault condition rather than reporting an actual temperature. This helps in preventing incorrect actions based on faulty sensor data. Therefore, rather than reporting an extreme temperature, the microprocessor might indicate a sensor failure or a circuit error.
