Resolving TIA Portal V13 SP1 "Error in Lower Level Component" on S7-300 / PLCSIM

The diagnostic message "Error in lower level component" in TIA Portal V13 SP1 (and the closely related "Error in low level component") is one of the most ambiguous alarms an S7-300 / ET 200 engineer encounters. The portal-level wording hides the real fault, which always lives one or two layers below the project tree: a module that does not match the offline configuration, a PLCSIM instance that presents a substitute article number, a decentralised ET 200 station that the CPU cannot reach, or a project in which compile results are stale. This reference walks through diagnosis, root-cause isolation, and verified corrective actions for the S7-300 CPU 315-2 PN/DP (MLFB 6ES7 315-2EH14-0AB0) running against PLCSIM Classic V5.4.5 (MLFB 6ES7 841-0CC05-0YA5) under TIA Portal V13 SP1 Update 9.

1. Problem Description

Engineer A loads an S7-300 project that was originally compiled and downloaded in TIA Portal V13 SP1. The hardware configuration contains a CPU 315-2 PN/DP (6ES7 315-2EH14-0AB0) and a small number of central SM 300 digital and analog modules. The intended test workflow is to use PLCSIM Classic to validate the user program before commissioning real hardware. The following symptoms are observed simultaneously or in sequence:

1. Download failure. A "Download to device" action terminates with the dialog Error in lower level component. No specific error code is shown in the dialog itself; the operation simply aborts.
2. Go online failure. Selecting "Go online" reports Error in lower level component. The project tree shows a red exclamation point on the PLC node and on each individual module underneath it.
3. CPU diagnostic detail. Double-clicking the PLC node reports: Offline and online are different. The modules below report: Not reachable.
4. Simulation reset requirement. Every fresh simulation requires a memory reset (MRES) before the new program can be loaded. Attempting to download a newer program over an existing one without MRES produces a system fault (SF) LED equivalent in PLCSIM.
5. Decentralised topology failure. When an ET 200S station is added to the project and PROFINET is wired to the CPU, the decentralised station reports Not found or the download again terminates with the lower-level error.

The combination of the first three symptoms, together with PLCSIM article number 6ES7 841-0CC05-0YA5 appearing in place of the real CPU article number, is the canonical fingerprint of the issue described in this article.

2. Architecture of the Fault

The phrase "lower level component" refers to the fact that the PLC device object visible in the TIA Portal project tree is itself a wrapper around the actual firmware and module objects that TIA Portal discovers during the online scan. The error originates at one of the following levels:

• Level 0 – PLCSIM compatibility layer. PLCSIM Classic 5.4.5 is implemented as a virtual module with article number 6ES7 841-0CC05-0YA5. TIA Portal V13 SP1 is fully aware of this article number, but the portal only matches it correctly if the simulation interface is selected in the PG/PC interface and the project is targeted at PLCSIM from the outset.
• Level 1 – Central rack mismatch. Inserted SM 300 modules may differ in firmware revision or article number from the offline configuration (for example a 6ES7 321-1BL00-0AA0 SM321 DI32x24V is present online but the offline configuration uses 6ES7 321-1BL80-0AA0). The portal accepts the substitution only if "Accept compatible module differences" is enabled, otherwise the rack stays in a fault state.
• Level 2 – Decentralised station. ET 200S, ET 200M, or ET 200SP stations attached via PROFINET or PROFIBUS report "station failure" if their device name, IP address, or slot configuration is not consistent with the offline project.
• Level 3 – Compile artefacts. A user program containing a syntax error, an I/O access error, or a wrong data block reference can force the download to fail even if the hardware configuration is clean. The lower-level error is, in this case, a misdirection.

The diagnostic tab in TIA Portal can usually distinguish between these levels; the online article number is the strongest single clue.

3. Diagnostic Procedure

Execute the steps below in order. Stop at the first step that yields a non-clean result and apply the corresponding fix from Section 4 before re-running the rest of the procedure.

1. Open the diagnostics view. Select the PLC node (PLC_1 [CPU 315-2 PN/DP]) in the project tree and switch to the Diagnostics tab in the lower section of TIA Portal. Confirm the exact alarm text and the slot/address reference.
2. Read the online article number. Compare the Online article number with the Offline article number. The expected online article numbers for this stack are:

   Component	Offline MLFB	Online MLFB (PLCSIM)
   CPU 315-2 PN/DP	6ES7 315-2EH14-0AB0	6ES7 841-0CC05-0YA5
   PLCSIM Classic	6ES7 841-0CC05-0YA5	6ES7 841-0CC05-0YA5

   If the online article number does not match one of the two expected values, the PG/PC interface is pointing at the wrong target.
3. Confirm the PG/PC interface assignment. Open Options → Set PG/PC interface. For PLCSIM Classic, select PLCSIM.S7-PLCSIM (PN/IE or MPI variant depending on topology) and ensure the access point S7ONLINE is bound to it. A common cause of the lower-level error is the interface defaulting to the physical Ethernet adapter while the project is configured for PLCSIM.
4. Check the diagnostics buffer. From the online view of the PLC, open Diagnostics → Diagnostics buffer and read the most recent events. Look for event IDs 0x1381 (rack failure), 0x3371 (PROFINET station failure), 0x35CE (module substituted), and 0x39B1 (I/O access error). Each event ID points to a specific fix in Section 4.
5. Run a full recompile. Right-click the PLC and select Compile → Hardware and software (rebuild all). Stale compile artefacts from a previous hardware configuration revision are a frequent cause of "Offline and online are different" even when the configuration screens are identical.
6. Capture the topology screenshot. In the Devices & Networks editor, export a graphical view of the topology with Tools → Graphical topology view and verify that every PROFINET device has the correct device name, IP address, and PROFINET device number.

4. Root Cause Catalogue and Targeted Solutions

4.1 PLCSIM article number is reported as a substitution

The online article number 6ES7 841-0CC05-0YA5 belongs to PLCSIM Classic 5.4.5 itself, not to the simulated CPU. When the offline project targets a real CPU 315-2 PN/DP (6ES7 315-2EH14-0AB0), the portal detects a module substitution and emits the "compatible module difference" diagnostic. This diagnostic, in itself, is informational and is supposed to be accepted automatically. The lower-level error appears only when the substitution is combined with one of the secondary causes below.

4.2 The PG/PC interface is set to the wrong adapter

If the S7ONLINE access point resolves to the physical Ethernet adapter of the engineering PC, the "Go online" scan attempts to ARP-resolve the CPU's PROFINET address. No real CPU is connected, so every module returns "Not reachable". The portal escalates the failure as "Error in lower level component" because the underlying WINAC/S7COMM stack reports the failure at the transport level.

4.3 Memory reset is required between downloads

PLCSIM 5.4.5 retains the previous program and retentive data across download attempts. When the new program differs in length or in the structure of instance DBs, the runtime hits a consistency check and the simulation enters a stop state. The portal-side effect is a System Fault during download, surfacing as the lower-level error on the next Go Online attempt.

1. In PLCSIM Classic, click MRES (or use Run → CPU → Memory reset).
2. Confirm the "STOP" and "SF" LEDs are off.
3. Re-issue Download to device from the portal.
4. If the new program relies on initial values for instance DBs, enable Download to device → Options → Initialize retentive data so the new structure is populated correctly.

4.4 Decentralised ET 200S station is not reachable

When an ET 200S or ET 200M station is added to the PROFINET topology without a CPU at the remote end (only the ET head module is required; the CPU is local, not remote), the station must have a unique PROFINET device name, a unique IP address, and must be assigned to the correct IO controller. The lower-level error in this case originates from event 0x3371 in the diagnostics buffer.

4.5 Compile error or I/O access error in the user program

The lower-level error can also be raised by the user program itself. A common case is an SFC/SFB call that addresses an I/O range that is not configured, or a fully qualified DB access that no longer matches the DB number. The diagnostics tab on the program blocks lists compile warnings that, while non-fatal at compile time, become fatal at download time when initial values are inconsistent.

1. Open Program blocks → Main [OB1] and inspect the cross-reference table for any I/O access highlighted in red.
2. Open the Compile output panel and resolve every warning that references I/O addresses.
3. Verify that any DB referenced from a FB has the same number of instance variables as the FB declaration. Version mismatches of FBs/DBs are a frequent cause of OB121 (programming error) at first scan, which in turn raises the lower-level alarm.

5. Step-by-Step Corrective Procedure

1. Close the TIA Portal project and stop PLCSIM if it is running.
2. Open Options → Set PG/PC interface. Select PLCSIM.S7-PLCSIM for the access point S7ONLINE. Click OK.
3. Re-open the project. Expand PLC_1 → Device configuration. Right-click the CPU and select Change device. Choose the matching simulation target. Click OK.
4. Right-click the CPU and select Compile → Hardware and software (rebuild all). Wait for the "Compile finished (no errors)" message.
5. Start PLCSIM Classic. Click MRES in PLCSIM to clear the simulated CPU.
6. In TIA Portal, select the PLC and click Download to device → Hardware and software. Confirm the device list shows PLCSIM and the correct CPU MLFB.
7. After the download, click Go online. The diagnostics tab should report OK for every module. The CPU should be in RUN with the green RUN LED.
8. Open the diagnostics buffer one more time and verify no event ID 0x1381, 0x3371, or 0x35CE remains.

6. Verification Matrix

7. Parameter Reference

8. Common Field Pitfalls and How to Avoid Them

The diagnostic label Error in lower level component tends to be over-interpreted as a hardware problem. In the S7-300 + PLCSIM + TIA Portal V13 SP1 stack it almost never indicates a defective module. The most frequent false leads are listed below in descending order of incidence.

Forgetting to re-target the project to PLCSIM. Real CPU MLFB 6ES7 315-2EH14-0AB0 remains in the offline project; the portal reports it as a substitute and raises the lower-level error. Solution: always check the offline/online MLFB pair as the first diagnostic step.

Selecting the wrong PG/PC interface. When the engineering PC has multiple Ethernet adapters (LAN, WLAN, virtual adapters for VPNs), TIA Portal may bind to the wrong one. Solution: lock the S7ONLINE access point to PLCSIM.S7-PLCSIM in the Windows control panel if necessary.

Not performing MRES between simulations. PLCSIM Classic does not auto-reset on download. Solution: train the workflow to always MRES before a new download.

Declaring an ET 200 station offline but expecting it in the runtime. Adding a station to the offline project without assigning a PROFINET name and IP address means the IO controller will never see the station. Solution: assign names from the topology editor before the first download.

Treating the warning as an error. The diagnostics tab reports "compatible module difference" as an information; users frequently click "Cancel" which causes the download to abort. Solution: click "Apply" or change the offline project to the simulation target.

9. Verification & Re-Test

After the corrective procedure, run a structured re-test. Add a small change to the user program (toggle an output in OB1) and perform a fresh download. Confirm:

• The download completes without prompting for MRES.
• The CPU transitions from STOP to RUN within 2 seconds.
• The watch table shows the toggling output and the inputs can be forced.
• Removing the ET 200 station from the offline project and recompiling should also succeed, confirming that the central configuration is independent of the decentralised station.

If any of the re-test steps fail, return to Section 3 and repeat the diagnostic procedure. The most common reason a re-test still fails is a stale compile cache; performing Project → Clean → Clean all and a full rebuild resolves the issue in 90 percent of the residual cases.

10. Related Siemens Documentation

• SIMATIC S7-300 CPU 315-2 PN/DP (6ES7315-2EH14-0AB0) product information
• SIMATIC S7-PLCSIM V5.4 SP5 manual
• TIA Portal V13 SP1 release notes and known issues
• S7-300 automation system system manual