W#16#38AA Siemens — CPU ↔ I/O module communication error (backplane bus)
At a glance
- Severity
- FAULT STOP
- Domain
- PLC
- Reference standard
- IEC 61131-2
- Product / context
- Module not accessible — S7-1500 backplane bus
What it means
The CPU cannot communicate with one or more I/O modules on the backplane bus (local rack). The module appears as unreachable in the hardware configuration. Variables associated with the module are forced to zero or show substitute values.
Probable causes
- I/O module not properly seated in the rack or backplane connector dirty/oxidized.
- Slot address in the hardware configuration not matching the physical module position.
- I/O module faulty or incompatible with the CPU firmware version.
- Rack power supply insufficient (power supply overloaded with too many installed modules).
Where to start
Identify the faulted module from the online diagnostics. Is the BF (Bus Fault) or SF (System Fault) LED on the module solid red?
Full guided diagnosis in the app
The diagnostic tree for this fault has 7 guided steps with measurements, checks and resolutions. You'll find it in ElectraIQ, one of the five IndexOS apps: free, works offline.
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The same fault on other brands
More Siemens codes
- 6EP1333-4BA00 — Unstable PLC power supply — cyclic restarts
- 6ES7521-1BL00-0AB0 — Digital input module — input not reading (stuck at 0)
- 6ES7522-1BL01-0AB0 — Digital output module — output not activating
- 6ES7531-7KF00-0AB0 — Analog input module — full-scale reading or error
- W#16#0049 — CPU in STOP
- W#16#35A3 — Memory error / backup battery depleted
- W#16#75D1 — Safety module F-CPU/F-DI/F-DO in passivation
- 0xE900 / 0xE901 — IO-Link communication lost (device not responding)