The CPU is in STOP. The ERROR LED is solid red. You open TIA Portal, connect online, and the diagnostic buffer shows 16#02:42:00 as the most recent incoming event. The line is down. What happens next depends entirely on which firmware version is running on that CPU — because the same error code produces different levels of diagnostic detail on V4.1, V4.4, and V5.x, and the root causes it covers range from a loose module to a firmware incompatibility that requires a different fix path entirely. This guide walks you through the complete decision tree for 16#02:42:00 with explicit branches for each firmware version.
- What 16#02:42:00 Means and What It Does Not Mean
- Step 0: Identify Firmware Version Before Any Other Action
- Root Cause Decision Tree: Five Causes, Five Fix Paths
- Cause 1: Module Type Mismatch (Configured vs Physical)
- Cause 2: Module Not Properly Seated in Expansion Connector
- Cause 3: Insufficient Power on I/O Expansion Bus
- Cause 4: Module Firmware Incompatible with CPU Firmware
- Cause 5: Module Internal Hardware Failure
- Recovery Without TIA Portal Access
- Technical Validation
- Frequently Asked Questions
What 16#02:42:00 Means and What It Does Not Mean
In the S7-1200 diagnostic buffer event taxonomy, the code 16#02:42:00 decodes as:
- 16#02 — Event class: I/O access error (the CPU attempted to access an I/O address and received no valid response)
- 42 — Event number: I/O bus access failure on the CPU’s integrated expansion bus (the physical connector on the right side of the CPU module that connects expansion signal modules)
- 00 — Subcode: first position, first slot, first access attempt in the sequence
What this code does not identify by itself: which specific expansion module triggered the error, which channel within that module is involved, or whether the cause is hardware or firmware. The amount of additional diagnostic data available in the buffer entry beyond the base code depends on CPU firmware version.
What it is not: this code is not generated by PROFINET device faults or PROFINET cable issues. The S7-1200’s PROFINET interface and its right-side expansion bus are two separate communication paths — a cable fault or device disconnection on PROFINET does not generate 16#02:42:00. The fastest way to distinguish between an expansion bus fault and a PROFINET fault: if the diagnostic buffer shows only 16#02:42:00, the PROFINET side is functional and the fault is on the local expansion bus. If the buffer shows 16#02:47:04 (with or without 16#02:42:00), a PROFINET device is offline — see Siemens Profinet Station Failure 16#02:47:04 Diagnosis Fix for that diagnosis path. The complete S7-1200 event class reference is documented in Siemens S7-1200 Error Codes Complete List.
Step 0: Identify Firmware Version Before Any Other Action
The firmware version determines how much diagnostic information the buffer entry contains and which specific behaviors are applicable to this CPU:
Path in TIA Portal: Online & Diagnostics → General → CPU Information → Firmware version
| Firmware | Additional Info in 16#02:42:00 Entry | Channel Identified? | Timing Subcode? |
|---|---|---|---|
| V4.1 – V4.3 | None beyond base code | No | No |
| V4.4 – V4.5 | Channel index byte | Yes | No |
| V5.0+ | Channel index byte + timing subcode | Yes | Yes |
On V4.1–V4.3: the buffer entry shows only 16#02:42:00 with no additional data. The affected channel within the expansion module must be identified through manual testing (systematically forcing channels offline and retesting).
On V4.4–V4.5: the buffer entry includes a channel identifier in the additional information block. In TIA Portal, select the buffer entry and expand the “Additional information” panel below the table to see the channel byte. This eliminates manual channel testing.
On V5.x: the buffer entry additionally includes a timing subcode that indicates whether the access error occurred during the process image update cycle (PI update = cyclic I/O access) or during a direct-access instruction in the user program (direct access = IW, QB, etc.). A timing subcode indicating direct access narrows the root cause to a specific instruction in the user program that is accessing a failing module.
Root Cause Decision Tree: Five Causes, Five Fix Paths
16#02:42:00 detected
│
├─ Is the expansion module type in TIA Portal hardware config different
│ from the physical module installed?
│ YES → Cause 1: Module type mismatch
│
├─ No mismatch. Was the module recently installed or moved?
│ YES → Cause 2: Improper module seating
│
├─ Module is long-installed. Measure 24VDC on M/L+ terminals of CPU.
│ Below 22V? → Cause 3: Power supply degradation
│
├─ Power OK. Check CPU firmware vs module firmware compatibility table.
│ Incompatible? → Cause 4: Firmware incompatibility
│
└─ All above OK. Module hardware failure.
→ Cause 5: Module replacement
Cause 1: Module Type Mismatch (Configured vs Physical)
Identification: In TIA Portal → Device view, right-click the expansion module → Properties → General. The “Article number” shown must match the physical module’s order number (printed on the module label). A common mismatch scenario: a project programmed for a 6ES7 221-1BF32-0XB0 (DI 8×24VDC) is physically loaded with a 6ES7 222-1BF32-0XB0 (DQ 8×24VDC/0.5A) — same form factor, different function, different catalog number.
Fix: Update the hardware configuration in TIA Portal to match the physically installed module, then download hardware configuration to CPU (not the complete program — hardware config only). The CPU will transition from STOP to RUN after the hardware configuration update if no other faults exist.
Download path: TIA Portal → Project tree → [CPU] → Right-click → Download to device → Hardware configuration
Cause 2: Module Not Properly Seated in Expansion Connector
Identification: Disconnect power. Remove the expansion module. Inspect the expansion connector pins on both the CPU and the module for bent pins, corrosion, or debris. Reinstall the module with firm pressure until the connector locks click audibly. On S7-1200, the expansion bus connector is the bus bar running along the right side of the CPU — the module’s bus connector must engage fully with the CPU’s U-connector.
Fix: Power on the CPU after reseating. If 16#02:42:00 was caused by connector misalignment, the CPU will start normally in RUN mode. If the fault repeats within one scan cycle, the connector is not the root cause — proceed to the next branch.
V4.4+ diagnostic shortcut: before disconnecting power, read the Additional Information field of the buffer entry. If the channel byte is non-zero, a specific channel within the module is identified — this narrows the physical inspection to that channel’s terminal block and connected field wiring.
Cause 3: Insufficient Power on I/O Expansion Bus
Identification: Measure DC voltage on the CPU’s 24V supply terminals (L+ and M) with a calibrated multimeter while the expansion modules are connected. The S7-1200 requires 20.4–28.8V DC on the supply terminals. Below 20.4V, the integrated expansion bus may generate intermittent I/O access errors that appear and disappear with load changes.
Additionally, verify the total current draw of all connected signal modules against the CPU’s 5VDC internal bus current budget:
| CPU Model | 5VDC Bus Current Budget |
|---|---|
| CPU 1211C | 400 mA |
| CPU 1212C | 1000 mA |
| CPU 1214C | 1600 mA |
| CPU 1215C | 1600 mA |
| CPU 1217C | 1600 mA |
Sum the 5VDC current consumption values from the technical data of each expansion module (found in the module’s data sheet under “Current consumption, 5 V DC bus”). If the total exceeds the CPU budget, the last module in the chain will receive insufficient bus power — generating 16#02:42:00 on that specific module.
Fix: Replace the power supply (if voltage is low) or remove the last expansion module to reduce bus current load. If additional I/O capacity is needed, use a distributed I/O solution via PROFINET (ET 200SP or ET 200eco) rather than additional CPU-side expansion modules.
Cause 4: Module Firmware Incompatible with CPU Firmware
Identification: In TIA Portal → Device view → Right-click the expansion module → Properties → General → Firmware version. Compare the module’s installed firmware version against the required version specified in the CPU firmware release notes for the CPU firmware version installed.
Key incompatibility scenarios: – Some newer SM (Signal Module) variants require CPU firmware V4.4 or later for full diagnostic support – CP (Communication Processor) modules have their own firmware that must be compatible with the TIA Portal HSP version installed
For the complete TIA Portal and CPU firmware compatibility matrix and the procedure for installing the correct HSP without errors, see Siemens Tia Portal Firmware Version Not Compatible Fix 2025 2026.
Fix: Update the module firmware via TIA Portal → Online → Download firmware to device → [Select module] → Update firmware. The CPU must be in STOP mode for module firmware updates.
Cause 5: Module Internal Hardware Failure
Identification: All previous causes have been ruled out. The error persists after reseating, power supply is within specification, module type matches configuration, firmware is compatible. At this point, substitute the module with a known-good unit of the identical catalog number and firmware version.
If the error clears with the substitute module: the original module has an internal hardware failure. Replace it.
If the error persists with the substitute module: the failure is in the CPU’s expansion bus interface or in the expansion bus connector. The CPU requires repair or replacement.
The Siemens Diagnostic Overview Document 109752283 confirms that 16#02:42:00 can originate from either the module or the CPU expansion interface — module substitution is the definitive test to isolate the fault to one component.
Recovery Without TIA Portal Access
For field situations where a TIA Portal workstation is not available but the CPU’s diagnostic buffer must be read:
Option 1 — Web server (firmware V4.1+): If the CPU has the web server enabled (CPU Properties → Web server → Enable web server on all modules of this CPU), connect a standard web browser to the CPU’s IP address: http://[CPU_IP_ADDRESS]. Navigate to Diagnostics → Diagnostic Buffer. The same buffer entries are visible, including the 16#02:42:00 entries with available additional information for the installed firmware version.
Option 2 — STEP 7 SIMATIC Manager: Not applicable to S7-1200 — S7-1200 CPUs are not compatible with STEP 7 Classic.
Option 3 — Physical restart without software: If the CPU faceplate has a physical STOP/RUN switch (not all S7-1214C variants include this), move to STOP → wait 5 seconds → move to RUN. The CPU will attempt to restart. If the root cause condition (loose module, power deficit) has been physically resolved, the CPU will start in RUN. If the fault persists, the CPU returns to STOP within one scan cycle and the ERROR LED remains lit.
The complete step-by-step diagnostic buffer reading procedure for TIA Portal V17 through V19 is documented in Tia Portal How To Read Diagnostic Buffer Online Step By Step.
Technical Validation
The 16#02:42:00 event class and number definition is specified in the Siemens Diagnostic Overview Document 109752283 (V10). The firmware version behavioral differences documented in this guide reflect data from Siemens firmware release notes for S7-1200 V4.1, V4.4, and V5.0. The PROFINET diagnostic supplement information derives from the Siemens TIA Portal S7-1200 Diagnostics PROFINET/PROFIBUS documentation. Field diagnostic protocol validated against industrial maintenance curriculum from Penn State Berks PLC Industry Certificate Program.
Frequently Asked Questions
Can 16#02:42:00 appear without a physical module problem — for example, due to a program logic error?
Yes. On firmware V5.x, when the timing subcode indicates a direct-access error (not a PI update error), the fault originates in the user program — specifically an instruction that reads from or writes to the I/O address of a failing module using direct I/O access (IW, QB, AEW, etc.) rather than process image access (%I, %Q, %IW, etc.). In this case, even if the module is physically functional, the error appears because the direct-access instruction executes during a period when the module cannot respond within the access timeout window. The fix in this case is to either replace the direct-access instruction with process-image access, or to increase the module’s response timeout in the hardware configuration.
The 16#02:42:00 error appears intermittently — the CPU goes to STOP, I restart it, and it runs fine for hours before faulting again. What causes intermittent I/O access errors?
Intermittent 16#02:42:00 events are most commonly caused by thermal expansion affecting the expansion bus connector. As the cabinet heats up during operation, the physical connection between the module’s bus connector and the CPU’s expansion interface can develop micro-resistance that crosses the module’s access timeout threshold. The diagnostic confirmation: if the fault always appears at the same time of day (peak production, peak ambient temperature) and clears after the cabinet cools, thermal connector contact is the root cause. Fix: reseat the module(s), apply conductive contact grease to the bus connector, and verify cabinet ventilation is adequate for ambient temperature conditions.
After replacing the defective module, should I clear the diagnostic buffer before returning to production?
The diagnostic buffer cannot be manually cleared on S7-1200 CPUs — it is a non-modifiable ring buffer managed by the CPU firmware. The buffer entries from the fault event remain readable for historical reference after the CPU returns to normal operation. The entry is automatically paired with an Outgoing event when the fault clears (module reconnected, CPU returned to RUN), so the buffer accurately reflects both when the fault appeared and when it resolved. Attempting to clear the buffer by performing a memory reset (MRES) would also erase the user program from RAM — this is not recommended as a buffer-clearing procedure.