XACT_E_ISOLATIONLEVEL (0x8004D008) Fix Guide

Cause #1: Your Code Asks for an Unsupported Isolation Level

This is the most common reason you're seeing 0x8004D008. Your application — typically a .NET or COM+ component — is requesting a transaction isolation level that Microsoft Distributed Transaction Coordinator (MSDTC) doesn't support. MSDTC supports only READ COMMITTED and SERIALIZABLE isolation levels. If you try to use READ UNCOMMITTED, REPEATABLE READ, or SNAPSHOT, you'll get this error.

This happens most often when you're using System.Transactions in C# or TransactionScope and you explicitly set the isolation level in code. SQL Server might accept it, but DTC won't when it escalates the transaction to a distributed one.

How to fix it

1. Open your project in Visual Studio (or your preferred IDE).
2. Search for TransactionScope or IsolationLevel in your codebase.
3. Look for lines like:
   

   using (var scope = new TransactionScope(TransactionScopeOption.Required, new TransactionOptions { IsolationLevel = IsolationLevel.ReadUncommitted }))

4. Change the isolation level to IsolationLevel.ReadCommitted or IsolationLevel.Serializable. For example:
   

   using (var scope = new TransactionScope(TransactionScopeOption.Required, new TransactionOptions { IsolationLevel = IsolationLevel.ReadCommitted }))

5. If you're not setting it explicitly, check your SQL queries. Sometimes SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED is run before the DTC transaction starts. Remove that line.
6. Rebuild and test your application. The error should be gone.

What to expect after fixing: Your app will no longer throw this error for this code path. DTC will use the default isolation level (usually READ COMMITTED) which it handles fine.

Cause #2: MSDTC Configuration Blocks the Isolation Level

Even if your code uses a supported isolation level, MSDTC might be configured in a way that forces an unsupported level. This is less common but I've seen it on locked-down servers or after a security policy update. The error shows up when you run a distributed transaction across two databases or servers.

How to fix it

1. Open Component Services: Press Win + R, type dcomcnfg, hit Enter.
2. In the left pane, expand Component Services > Computers > My Computer > Distributed Transaction Coordinator.
3. Right-click Local DTC and select Properties.
4. Click the Security tab.
5. Under Transaction Manager Communication, check these settings:
   
   • Allow Inbound must be checked.
   • Allow Outbound must be checked.
   • No Authentication Required is often needed for older apps (you can try Mutual Authentication Required first if security is strict).
6. Click OK. A warning will appear about restarting the service — click Yes.
7. Restart the Distributed Transaction Coordinator service:
   
   • Open Services (press Win + R, type services.msc, Enter).
   • Find Distributed Transaction Coordinator.
   • Right-click it, select Restart.
8. Also check the Network DTC Access setting. Under that same Security tab, ensure Allow Remote Clients is checked if your app connects from another machine.
9. After restarting, test your transaction again.

What to expect: DTC will now accept the standard isolation levels from remote callers. If the error persists, move to Cause #3.

Cause #3: COM+ Component Forces an Invalid Isolation Level

If your app uses COM+ components (common in older enterprise apps or VB6-based systems), the COM+ catalog might have an isolation level set that DTC rejects. This is the sneakiest cause because the error looks like it's from your code but it's actually the COM+ configuration.

I've seen this on Windows Server 2012 R2 and 2016 after a COM+ application is deployed with default settings that use ISOLATIONLEVEL_READUNCOMMITTED.

How to fix it

1. Open Component Services again (dcomcnfg).
2. Navigate to Component Services > Computers > My Computer > COM+ Applications.
3. Find the COM+ application that's throwing the error. If you're unsure, look for the one that matches your app's name or component DLL.
4. Right-click the COM+ application, select Properties.
5. Click the Transactions tab.
6. Look at the Transaction Isolation Level dropdown. It's usually set to Serialized by default. If it's set to anything else (like Read Uncommitted or Repeatable Read), that's your problem.
7. Change it to Serialized (which maps to SERIALIZABLE) or Read Committed. Serialized is safest.
8. Click OK. COM+ will prompt you to shut down the application — click Yes.
9. Restart your application service or IIS (if it's a web app) to pick up the change.

What to expect: After this change, the COM+ component will request a supported isolation level, and DTC will stop complaining.

Quick-Reference Summary Table

If you've tried all three and still get the error, check your firewall for port 135 (RPC Endpoint Mapper) and the dynamic RPC port range (usually 49152–65535). Blocked ports can cause DTC to fail with this error in rare cases. But 9 times out of 10, it's one of the three fixes above. Good luck.