Fix SimConnect errors in FSX and FSX: Steam Edition using the right legacy installers, repair order and add-on connection checks.
Fix SimConnect errors in FSX by repairing the SimConnect runtime that your add-on expects, not by downloading a loose SimConnect.dll. In FSX: Steam Edition, verify the game files and run the included legacy SimConnect installers. For boxed FSX, repair the simulator and restore its correct service pack or Acceleration update path.
Which SimConnect error are you seeing?
The symptom usually reveals whether FSX itself has a damaged runtime or whether one particular add-on is incompatible or misconfigured.
| Symptom | Likely cause | Best first action |
|---|---|---|
side-by-side configuration is incorrect | A required SimConnect or Visual C++ assembly is missing | Check the Windows SideBySide event and repair the named component |
SimConnect.dll was not found | The expected legacy runtime is absent, or the add-on was installed incorrectly | Run the bundled SimConnect installer, then repair the add-on |
| Every external tool reports that it cannot connect | SimConnect is damaged, FSX is not fully loaded, or a custom configuration is invalid | Repair the runtimes and test with a local default connection |
| Only one add-on fails | Wrong add-on edition, installation path or client version | Check that add-on rather than reinstalling every SimConnect package |
| Local tools work but a networked client fails | Address, protocol, port or firewall mismatch | Check the host and client configuration files |
For a side-by-side error, open Event Viewer and inspect Windows Logs > Application for a SideBySide entry recorded at the failure time. If it names Microsoft.FlightSimulator.SimConnect, repair that SimConnect generation. If it names Microsoft.VC80.CRT, repair the matching x86 Visual C++ runtime supplied with FSX or the add-on. Our steps for tracing missing FSX side-by-side assemblies cover this diagnosis in detail.
How do I repair SimConnect in FSX: Steam Edition?
FSX: Steam Edition includes three legacy SimConnect installers because older add-ons may request the RTM, SP1 or Acceleration-era interface specifically.
- Close FSX and every connected utility. This includes weather engines, moving maps, hardware bridges and flight-recording tools.
- Verify the FSX installation in Steam. Open the game's properties, select the installed-files section and verify the files. This restores missing installer files but may not register every legacy runtime automatically.
- Open the FSX installation folder. From there, go to
SDK > Core Utilities Kit > SimConnect SDK > LegacyInterfaces. - Run each bundled installer. Open the
FSX-RTM,FSX-SP1andFSX-XPACKfolders and run theSimConnect.msifound in each. Choose Repair if that option appears; otherwise complete the installation normally. - Test after loading a flight. Start FSX, reach the cockpit and only then launch the external client. This eliminates failures caused by a tool attempting to connect before the simulator is ready.
- Repair the individual add-on if it alone still fails. Use the installer intended for Steam Edition and confirm that it detected the actual Steam library path. Our guide to Steam Edition add-on compatibility and installation paths explains the common edition mismatch.
The three legacy packages are designed to coexist. Do not uninstall one merely because its version number is lower, and do not apply boxed-FSX service packs to Steam Edition.
How do I repair SimConnect in boxed FSX?
Boxed FSX receives its SimConnect assemblies through the base simulator, service packs and Acceleration, so the repair must follow the update path for your edition.
- Back up customised files. Preserve aircraft, scenery and configuration changes before running a simulator-level repair.
- Repair the base FSX installation. Use the original media's setup programme if it offers a repair option. Reinstallation should be the last resort, not the first.
- Restore the proper update level. A non-Acceleration installation normally uses SP1 followed by SP2. Acceleration users should repair or reinstall Acceleration rather than placing standalone SP2 over it. Use our edition-specific FSX service-pack and update sequence if the installed state is uncertain.
- Run FSX before reinstalling add-ons. Let the simulator complete its first launch, close it, then repair the affected add-on so its installer sees the restored FSX components.
Several entries named Microsoft Flight Simulator SimConnect Client may appear in Windows' installed-app list. That is normal: add-ons compiled against different FSX generations need those versions side by side. Installing the full FSX SDK is not normally required just to run an add-on.
Why does an add-on still fail after repairing SimConnect?
If other SimConnect clients work, the remaining fault belongs to the affected add-on, its launch entry or its connection settings rather than the shared FSX runtime.
Local add-ons and external tools
- Load an aircraft and enter the cockpit before starting the client.
- Run FSX and the external programme at the same Windows privilege level. Avoid running one as administrator while the other runs normally.
- Do not create
SimConnect.xmlorSimConnect.cfgfor an ordinary same-PC connection unless the add-on explicitly requires them. - If an add-on no longer starts automatically, inspect
exe.xmlanddll.xmlin the active FSX configuration folder. Rename one file at a time for testing rather than deleting it. Boxed FSX commonly uses theFSXfolder under AppData, while a side-by-side Steam installation may useFSX-SE.
For a concrete external-client test sequence, follow our worked SimConnect connection checks for local utilities.
Remote SimConnect connections
A client on another computer needs matching network settings: the FSX host's SimConnect.xml and the client's SimConnect.cfg must agree on protocol, address and port. Allow FSX and the chosen port through the private-network firewall, and check whether the host computer's local IP address has changed.
Loose DLL files
Never download a random SimConnect.dll or copy one into the FSX root, System32 or SysWOW64. SimConnect uses versioned assemblies and installer registration; a mismatched loose DLL can replace one visible error with crashes or further side-by-side failures. A DLL deliberately supplied inside an add-on's own folder should be managed by that add-on's installer.