SimConnect_Open consistently returns E_FAIL in MSFS 2024 1.7.35.0

[MSFS 2024] Bug Reports SimConnect
1

Version: Microsoft Flight Simulator 2024 1.7.35.0 (Microsoft Store)
SDK: Microsoft Flight Simulator 2024 SDK Core 1.6.9
Platform: Windows 11 x64, Microsoft Store / Xbox app
Frequency: Consistently (100%)
Severity: Blocker for external SimConnect clients

Bug description:
Every external SimConnect client is rejected by SimConnect_Open with HRESULT
E_FAIL, even after loading a free flight with the default Cessna 172. The issue
also occurs with a minimal client built against the official MSFS 2024 SDK.

Official SDK test result:
FAILED: COMException: HRESULT E_FAIL was returned from a call to a COM component.

The same connection attempt through Python-SimConnect fails at SimConnect_Open
with WinError -2147467259 / E_FAIL. The FlightSimulator2024 process is running
and a free flight is fully loaded when the test is performed.

Reproduction steps:

  1. Start MSFS 2024 version 1.7.35.0.
  2. Load a free flight with the default Cessna 172 and wait until the cockpit is usable.
  3. Start a minimal x64 .NET Framework client using the official SDK assembly and native SimConnect.dll.
  4. Construct Microsoft.FlightSimulator.SimConnect.SimConnect with config index 0.
  5. SimConnect_Open immediately returns HRESULT E_FAIL.

Troubleshooting already completed:

  • Installed MSFS 2024 SDK Core 1.6.9 from Developer Mode.
  • Installed the official SimConnect.msi runtime included with the SDK.
  • Tested both the official SDK DLL and the legacy MSFS 2020-compatible DLL.
  • Regenerated SimConnect.xml; MSFS generated a clean default configuration.
  • Confirmed MSFS was fully loaded into the default Cessna 172.
  • Disabled every EXE.xml auto-start add-on and restarted MSFS.
  • Verified no FSRealistic, AutoFPS, Fenix, Couatl, Aerosoft, Simlink,
    LandingToast, or UniversalAnnouncer process was running during the clean test.
  • Repaired MSFS through Windows Installed Apps.
  • Ran Verify and Repair through the Xbox app.
  • Checked Microsoft Store and Windows Update, then rebooted Windows.
  • The failure is unchanged after all steps above.

Expected result:
SimConnect_Open returns S_OK and the external client receives the open event.

Actual result:
SimConnect_Open returns HRESULT E_FAIL before any data definitions or simulation
variables are requested.

2

If you are using managed code, include the

Microsoft.FlightSimulator.Simconnect.dll

alongside your executable

(Just to make sure Simconnect is working properly, run one of the Simconnect samples, they are in C++ but you just need to click play on visual studio)

3

Thank you. I verified that Microsoft.FlightSimulator.SimConnect.dll and the native SimConnect.dll are both located directly next to the managed probe executable.

I have now also built and run a minimal native x64 C++ client directly against the official MSFS 2024 SDK files:

  • SimConnect SDK/include/SimConnect.h
  • SimConnect SDK/lib/SimConnect.lib
  • SimConnect SDK/lib/SimConnect.dll placed next to the executable

The only operation performed by the native client is SimConnect_Open while MSFS 2024 is fully loaded into the default Cessna 172 cockpit.

Result:
SimConnect_Open HRESULT: 0x80004005
FAILED: Native C++ client could not connect.

Therefore the same E_FAIL occurs with native C++, independently of .NET and Python.

The installed MSFS 2024 SDK Core 1.6.9 does not contain the C++ sample projects on this system; the SimConnect SDK folder contains only include, installer, lib and VS property files. If the samples require a separate SDK component, please let me know where it can be installed from.

4

Confirm, there is a separate download for Samples,
You can reach them via the Help menu in game

5

Hello @Wilson7645

I suspect this msi is installing obsolete version of the dlls.
I will have this reviewed.

You do not need to install it, you should rather use headers and libraries located in C:\MSFS 2024 SDK\SimConnect SDK include and lib directories instead.

Have a look at SimVarWatcher for C# project configuration, and SimConnect samples for C++ project configuration.
They are all provided with the Samples installer.

Regards,
Sylvain

6

Hi Sylvain,

Thank you for the clarification. Although I installed the SimConnect.msi during troubleshooting, the minimal probes were built directly against the current MSFS 2024 SDK Core 1.6.9 files, not against an MSI-installed copy:

  • C#: C:\MSFS 2024 SDK\SimConnect SDK\lib\managed\Microsoft.FlightSimulator.SimConnect.dll
  • Native runtime DLL: C:\MSFS 2024 SDK\SimConnect SDK\lib\SimConnect.dll
  • Native C++: headers and import library from the SDK include and lib directories

I also verified that every SimConnect.dll copied beside the test executables has the same SHA-256 hash as the DLL in the SDK lib directory. I tested .NET 8 x64, .NET Framework x64, and a minimal native C++ client. All of them still fail immediately in SimConnect_Open with HRESULT E_FAIL while a free flight is fully loaded.

Therefore the failure also occurs when using the SDK files directly and does not appear to depend on the potentially outdated MSI runtime. If useful, I can provide the minimal source projects and exact build commands for comparison with SimVarWatcher and the SDK samples.

Best regards,
Wilson7645

7

Hello @Wilson7645

So the SimVarWatcher and C++ SimConnect samples do not fail in a similar way?

Yes please.

Regards,
Sylvain

8

Hi Sylvain,

To be precise: I have not yet rerun the official SimVarWatcher and the official C++ OpenAndClose sample during the same simulator session, so I cannot honestly claim their current result. The failures reported so far came from my own minimal probes using the SDK files directly.

Native C++ x64

#include <Windows.h>
#include <SimConnect.h>
#include <iomanip>
#include <iostream>

int main() {
    HANDLE connection = nullptr;
    const HRESULT result = SimConnect_Open(
        &connection, "CARTANA native SDK probe", nullptr, 0, nullptr, 0);
    std::cout << "SimConnect_Open HRESULT: 0x" << std::hex
              << std::uppercase << static_cast<unsigned long>(result) << std::endl;
    if (SUCCEEDED(result)) {
        SimConnect_Close(connection);
        return 0;
    }
    return 1;
}

Built from an x64 Native Tools Command Prompt for VS 2022:

cl /nologo /EHsc /W4 /O2 /I"C:\MSFS 2024 SDK\SimConnect SDK\include" simconnect_native_probe.cpp /Fe:simconnect_native_probe.exe /link /LIBPATH:"C:\MSFS 2024 SDK\SimConnect SDK\lib\static" SimConnect.lib Shlwapi.lib User32.lib Ws2_32.lib

Observed output with a free flight fully loaded:

SimConnect_Open HRESULT: 0x80004005

C# / .NET 8 x64

using Microsoft.FlightSimulator.SimConnect;

try
{
    using var signal = new AutoResetEvent(false);
    using var sim = new SimConnect(
        "CARTANA MSFS 2024 Probe", IntPtr.Zero, 0, signal, 0);
    Console.WriteLine("CONNECTED");
}
catch (Exception ex)
{
    Console.WriteLine($"FAILED: {ex.GetType().Name}: {ex.Message}");
}

The x64 project references these SDK files directly:

C:\MSFS 2024 SDK\SimConnect SDK\lib\managed\Microsoft.FlightSimulator.SimConnect.dll
C:\MSFS 2024 SDK\SimConnect SDK\lib\SimConnect.dll

Build and run:

dotnet clean .\msfs_probe.csproj -c Release
dotnet build .\msfs_probe.csproj -c Release
.\bin\Release\net8.0-windows\msfs_probe.exe

Observed result:

FAILED: COMException: Error HRESULT E_FAIL has been returned from a call to a COM component.

The copied runtime DLL and the SDK lib\SimConnect.dll both have SHA-256:

9EB25BF07371FB8F36494AE609BAFB12FB0F8DC64EF966361CD5080CE26F6E66

I will rerun the installed official SDK SimVarWatcher and SimConnectSamples\OpenAndClose against the same fully loaded free-flight session and report their results separately.

Best regards,
Wilson7645

9

I’d say you probably want to check this first so we can determine if the issue has more to do with your environment or is specific to your projects.

Regards,
Sylvain

10

Hi Sylvain,

I have now tested both unchanged official SDK programs during the same fully loaded free-flight session in MSFS 2024 1.7.35.0. The FlightSimulator2024 process remained responsive throughout.

  1. Official C++ sample:
    C:\MSFS 2024 SDK\Samples\VisualStudio\SimConnectSamples\OpenAndClose\x64\Release\OpenClose.exe

Result:

Failed to connect to Flight Simulator
  1. Official SimVarWatcher:
    C:\MSFS 2024 SDK\Samples\VisualStudio\SimvarWatcher\bin\x64\Release\Simvars.exe

The official UI opened normally. After pressing Connect, the SimVarWatcher window disappeared. The Simvars process remained running and responsive but had no top-level window, so I terminated only that hung test process. MSFS itself remained open and responsive.

Therefore both unchanged official SDK samples also fail in the same simulator session. This confirms that the issue is not specific to CARTANA or to my minimal projects; it appears to be related to the simulator installation/runtime environment or this MSFS build.

Best regards,
Wilson7645

11

It is not caused by the build either as you appear to be the only one reporting this issue so far.
I would suggest you look at your Windows configuration and any other firewall on your side.

By default, SimConnect relies on Named Pipes but a firewall can block those.
Alternatively, you can use SimConnect.cfg to select another protocol and see if you still have the issues.

A tool like Wireshark will probably help you confirm this is the issue.
(There may be some other tool to test Named Pipes that I’m not aware of)

Regards,
Sylvain

12

Hi Sylvain,

Thank you. I tested the alternative transports and checked the local firewall state while the same free flight was fully loaded.

The active server configuration is:
C:\Users\Admin\AppData\Local\Packages\Microsoft.Limitless_8wekyb3d8bbwe\LocalCache\SimConnect.xml

It defines:

  • IPv4, local scope, port 500
  • IPv6, local scope, port 501
  • pipe Custom\SimConnect
  • the default FSX pipe

Named-pipe checks:

  • \.\pipe\Custom\SimConnect exists
  • \.\pipe\Microsoft Flight Simulator\SimConnect exists
  • WaitNamedPipeW reports both pipes available to the current user (WinError 0)

Alternative-protocol tests:
I copied the unchanged official OpenClose.exe to isolated test folders (the SHA-256 hash still matched the SDK binary) and placed a side-by-side SimConnect.cfg beside it.

IPv4 test:
[SimConnect.0]
Protocol=IPv4
Address=127.0.0.1
Port=500
MaxReceiveSize=41088
DisableNagle=1

IPv6 used Protocol=IPv6, Address=::1, and Port=501.

Both tests still returned:
Failed to connect to Flight Simulator

During the same session, Windows showed no TCP listener on port 500 or 501, despite those static endpoints being present in SimConnect.xml.

Windows Firewall is enabled on all profiles. I found no matching Defender or Windows Firewall block event for FlightSimulator2024, OpenClose, Simvars, SimConnect, port 500, or port 501 during the tests. Only Microsoft Defender was running; no third-party firewall or antivirus process was detected.

So the named pipes are present and available, but the official client still cannot complete SimConnect_Open; forcing IPv4 or IPv6 also fails, and the simulator does not appear to open the configured TCP listeners.

Is there a SimConnect server-side log or another diagnostic flag in MSFS 2024 that could show why the server rejects or fails to initialize these connections?

Best regards,
Wilson7645

13

The path to the log file is configured in SimConnect.ini, and you should find a SimConnect.log file by default.

Regards,
Sylvain

14

Hi Sylvain,

Good news: the issue is no longer reproducible after a complete MSFS restart.

After placing SimConnect.ini in the Microsoft Store LocalCache folder and then fully restarting MSFS 2024, the unchanged official OpenClose.exe connected and disconnected successfully four consecutive times:

Connected to Flight Simulator!
Disconnected from Flight Simulator

My CARTANA SimConnect client now also connects successfully and reads live C172 telemetry, including altitude, airspeed, heading, vertical speed, on-ground state, flaps, and autopilot data. We tested it during an active flight and the values update correctly.

One detail: no SimConnect*.log file was created even with level=Verbose and an explicit file path in SimConnect.ini. Therefore I cannot confirm whether the INI changed server initialization or whether the full restart simply cleared a transient server state.

At this point SimConnect itself is working reliably again. Thank you very much for guiding me through the official-sample and transport checks.

Best regards,
Wilson7645