The first step in determining the problem and it's solution is to take note of when the Error Code C533: HEATUP MODE is appearing. Once you know when the error code is appearing, scroll down & locate the proper chart for solutions
If after selecting a cycle, the display reads FILLING and then advances to the heat up mode prior to giving you the C533: HEATUP MODE Error Code, then this is what you need to look for:
High-limit thermostat opened for at least 0.25 seconds during POWER UP mode