The first step in determining the problem and it's solution is to take note of when the Error Code C633: HEATUP MODE is appearing. Once you know when the error code is appearing, scroll down & locate the proper chart for solutions
If, after you select a cycle and push start, the autoclave does not display filling, but goes directly into the heat up mode, then this is what you need to look for
This will occur if chamber pressure does not reach 1 psi within one minute after air valve closes; or if chamber pressure does not reach 4.93 psi within 25 minutes after starting the HEAT UP mode
If after selecting a cycle, the display reads FILLING and then advances to the heat up mode prior to giving you the C633: HEATUP MODE Error Code, then this is what you need to look for: