Siemens SIMATIC S7-1500 System Manual page 177

Application planning
6.4 Redundancy scenarios
Diagnostics
System state, operating states and error displays after primary-backup switchover:
• Redundant system → RUN-Solo system state
• Primary CPU → RUN operating state
– MAINT LED → yellow light: The R/H system is not in the RUN-Redundant system state.
• Backup CPU → failed
Note
To get detailed diagnostics information, evaluate the diagnostics buffer.
Solution
Replace the defective CPU. You can find additional information on the procedure in the
section Replacing defective R/H CPUs (Page 446).
176
No partner CPU has been found in the redundant system. The PROFINET ring is open.
S7-1500R/H redundant system
System Manual, 01/2024, A5E41814787-AF