I have measured a high reading x. Is this normal?
Apple doesn't publish any specifications for the temperature values measured at individual sensor locations. In most cases however, the answer is "yes". All up-to-date Macintosh models automatically shut down or enter sleep mode if one of the readings exceeds a critical limit. For this reason it is impossible that your computer can be damaged in an overheat condition.
Regarding concrete temperature values the following facts are known:
For computers with G5 processors, the design limits for temperature and fan speeds can be read out by using the MPU information panel of the application. The maximum on-chip temperature for the PPC 970 is 85 degrees Celsius (158 degrees Fahrenheit), for the PPC 970FX it is 105 degrees Celsius (221 degrees Fahrenheit).
The cooling system of the "Power Mac G4 (Mirrored Drive-Doors)" is designed to hold the temperature of the processor board in a range of 58 to 60 degrees Celsius (136..140 degrees Fahrenheit). Under full load the values can go up to 70 degrees Celsius (158 degrees Fahrenheit). After that point, the critical limit has been reached.
Most older G3 or G4 processors are designed for a maximum operating temperature of 105 degrees Celsius (221 degrees Fahrenheit).
G4 processors of type 7455 or later (typically used in Macintosh systems with 867 MHz and above) are designed for a maximum operating temperature of 65 degrees Celsius (149 degrees Fahrenheit). This limit applies when directly measuring the on-die temperature. Because the sensors are often mounted in some distance from the processor and measure cache memory temperature at the same time, the displayed value can be higher than the value measured at the actual chip die.
