How do I fix the side-by-side configuration error in FSX?

In FSX and FSX: Steam Edition, this error means Windows cannot load a required side-by-side assembly, usually an x86 Visual C++ runtime or a legacy SimConnect component. Identify the missing assembly in Event Viewer or with sxstrace, then repair that exact prerequisite; do not download individual DLLs or remove newer runtimes.

Why does FSX show a side-by-side configuration error?

Windows builds an activation context from the executable's manifests and the assemblies installed in its component store. The error appears when a required assembly is missing, damaged, the wrong architecture, or referenced by an invalid manifest.

First establish which executable is failing. If fsx.exe will not open, concentrate on FSX's prerequisites and installation files. If FSX starts but a weather utility, traffic program, aircraft installer or launcher fails, that add-on usually owns the missing dependency; our guide to checking boxed-FSX and Steam Edition add-on dependencies can help distinguish the two cases.

How do I identify the missing FSX component?

Event Viewer normally reveals the required runtime or manifest, removing the need to install packages at random.

1. Reproduce the error once. Note whether it appears when starting FSX itself or a separate add-on executable.
2. Open Event Viewer. Press Windows+R, enter eventvwr.msc, then open Windows Logs and Application.
3. Find the matching SideBySide event. Look at entries created at the exact failure time. The General tab may name an assembly such as Microsoft.VC80.CRT, Microsoft.VC90.CRT or Microsoft.FlightSimulator.SimConnect.
4. Use sxstrace if the event is vague. Open Command Prompt as administrator and run sxstrace trace -logfile:%TEMP%\fsx-sxs.etl. Reproduce the error, run sxstrace stoptrace, then run sxstrace parse -logfile:%TEMP%\fsx-sxs.etl -outfile:%TEMP%\fsx-sxs.txt.
5. Read the first unresolved reference. Open the generated text file and look for the missing assembly, architecture, version or manifest syntax error. Later warnings may be consequences rather than the original fault.

Which Visual C++ redistributable does FSX need?

Install the runtime identified by the SideBySide record, not simply the newest Visual C++ package. FSX is a 32-bit application, so it normally needs the x86 package even on 64-bit Windows; installing only the x64 equivalent does not satisfy that dependency.

VC80 refers to the Visual C++ 2005 generation, while VC90 refers to 2008. Later redistributables do not replace these older side-by-side assemblies, and several generations are meant to coexist.

1. Try Repair first. In Windows' installed-apps list or Programs and Features, select the matching Visual C++ redistributable and use Change or Repair if offered.
2. Reinstall only the indicated package if necessary. Use a Microsoft-signed installer supplied with FSX, its installation media, Steam's prerequisite files or the affected add-on.
3. Restart Windows and test the same executable. If the SideBySide event changes, inspect the new missing reference rather than assuming the original repair failed.

Do not uninstall every Visual C++ package before doing this. Other simulators, aircraft and utilities may depend on those parallel versions.

What if the error names SimConnect?

A SimConnect side-by-side error usually belongs to an FSX client utility rather than fsx.exe itself. Legacy SimConnect versions coexist intentionally, so install the particular version expected by the utility instead of replacing it with one arbitrary DLL.

Use the legacy SimConnect installer supplied with the add-on or the relevant installer under the FSX SDK's LegacyInterfaces area when that folder is present. Our explanation of how installed SimConnect assemblies differ from copied client DLLs covers the safe repair principle; for FSX, use the legacy package identified by the trace.

How do I repair FSX after fixing the prerequisite?

Repair the simulator files only when the trace points to FSX itself, a stock manifest or a damaged installation.

• FSX: Steam Edition: Open FSX's Steam properties, use the installed-files option to verify file integrity, then launch it once through Steam so prerequisite setup can run. Back up any manually modified default files first.
• Boxed FSX: Run Repair from the matching installation media. Restore the correct SP1, SP2 or Acceleration level afterwards; our FSX service-pack installation order explains which updates belong together.
• Moved or reinstalled boxed copy: If setup cannot find FSX or targets an obsolete folder, correct the installation path using our boxed-FSX registry repair procedure before running another repair.

Will reinstalling FSX fix the error?

A full FSX reinstall is rarely the first fix because reinstalling the simulator may leave the broken Windows runtime or SimConnect assembly untouched. Use it only after the trace identifies damaged FSX files and a normal repair or Steam verification has failed.

If several unrelated applications also produce side-by-side errors, the Windows component store may be damaged. On Windows 10 or 11, run DISM /Online /Cleanup-Image /RestoreHealth from an administrator Command Prompt, followed by sfc /scannow, then restart Windows before repairing the identified runtime again.

Which side-by-side fixes should I avoid?

• Do not download loose copies of msvcr, msvcp or SimConnect.dll and place them in the FSX, System32 or SysWOW64 folders.
• Do not delete files or manifests from WinSxS.
• Do not assume the latest Visual C++ package replaces the older version named in the event.
• Do not uninstall all redistributables or install every package you can find.
• Do not rely on compatibility mode or “Run as administrator”; neither supplies a missing assembly.