diff --git a/src/CsvColumnizer/CsvColumnizer.cs b/src/CsvColumnizer/CsvColumnizer.cs
index e780fd2c..b5e0c8c8 100644
--- a/src/CsvColumnizer/CsvColumnizer.cs
+++ b/src/CsvColumnizer/CsvColumnizer.cs
@@ -176,6 +176,25 @@ public void Selected (ILogLineMemoryColumnizerCallback callback)
     {
         ArgumentNullException.ThrowIfNull(callback, nameof(callback));
 
+        DetectDelimiterFromFile(callback);
+        BuildColumns(callback);
+    }
+
+    /// <summary>
+    /// This instance may not have pre-processed the current file (e.g. a clone that loaded the saved config),
+    /// so re-detect the delimiter from the file's first line, like PreProcessLine() does on load.
+    /// </summary>
+    private void DetectDelimiterFromFile (ILogLineMemoryColumnizerCallback callback)
+    {
+        var line = _firstLine ?? callback.GetLogLineMemory(0);
+        if (line != null)
+        {
+            AutoDetectDelimiter(line.FullLine);
+        }
+    }
+
+    private void BuildColumns (ILogLineMemoryColumnizerCallback callback)
+    {
         if (_isValidCsv) // see PreProcessLine()
         {
             _columnList.Clear();
@@ -183,28 +202,33 @@ public void Selected (ILogLineMemoryColumnizerCallback callback)
                 ? _firstLine ?? callback.GetLogLineMemory(0)
                 : callback.GetLogLineMemory(0);
 
+            string[]? fields = null;
             if (line != null)
             {
-                using CsvReader csv = new(new StringReader(line.FullLine.ToString()), _config.ReaderConfiguration);
-                _ = csv.Read();
-                _ = csv.ReadHeader();
-
-                var fieldCount = csv.Parser.Count;
-
-                var headerRecord = csv.HeaderRecord;
+                try
+                {
+                    fields = ReadFields(line.FullLine.ToString());
+                }
+                catch (CsvHelperException)
+                {
+                    // first line doesn't parse with the current settings (e.g. wrong delimiter); same fallback as SplitCsvLine
+                }
+            }
 
-                if (_config.HasFieldNames && headerRecord != null)
+            if (fields != null)
+            {
+                if (_config.HasFieldNames)
                 {
-                    foreach (var headerColumn in headerRecord)
+                    foreach (var headerColumn in fields)
                     {
                         _columnList.Add(new CsvColumn(headerColumn));
                     }
                 }
                 else
                 {
-                    for (var i = 0; i < fieldCount; ++i)
+                    for (var i = 0; i < fields.Length; ++i)
                     {
-                        _columnList.Add(new CsvColumn("Column " + i + 1));
+                        _columnList.Add(new CsvColumn(string.Format(CultureInfo.InvariantCulture, "Column {0}", i + 1)));
                     }
                 }
             }
@@ -254,6 +278,12 @@ public void Configure (ILogLineMemoryColumnizerCallback callback, string configD
         var configPath = configDir + "\\" + CONFIGFILENAME;
         FileInfo fileInfo = new(configPath);
 
+        // show the file's delimiter rather than the one from the saved config
+        if (callback != null)
+        {
+            DetectDelimiterFromFile(callback);
+        }
+
         CsvColumnizerConfigDlg dlg = new(_config);
 
         if (dlg.ShowDialog() == DialogResult.OK)
@@ -268,7 +298,11 @@ public void Configure (ILogLineMemoryColumnizerCallback callback, string configD
 
             _config.ConfigureReaderConfiguration();
 
-            Selected(callback);
+            // no re-detection here: keep the delimiter the user chose in the dialog
+            if (callback != null)
+            {
+                BuildColumns(callback);
+            }
         }
     }
 
@@ -360,6 +394,15 @@ private void AutoDetectDelimiter (ReadOnlyMemory<char> lineContent)
         }
     }
 
+    /// <summary>
+    /// Parses one line into its fields; avoids ReadHeader(), which throws when HasHeaderRecord is false.
+    /// </summary>
+    private string[]? ReadFields (string line)
+    {
+        using CsvReader csv = new(new StringReader(line), _config.ReaderConfiguration);
+        return csv.Read() ? csv.Parser.Record : null;
+    }
+
     private ColumnizedLogLine SplitCsvLine (ILogLineMemory line)
     {
         if (line.FullLine.IsEmpty)
@@ -374,12 +417,7 @@ private ColumnizedLogLine SplitCsvLine (ILogLineMemory line)
 
         try
         {
-            using CsvReader csv = new(new StringReader(line.FullLine.ToString()), _config.ReaderConfiguration);
-            _ = csv.Read();
-            _ = csv.ReadHeader();
-
-            //we only read line by line and not the whole file so it is always the header
-            var records = csv.HeaderRecord;
+            var records = ReadFields(line.FullLine.ToString());
 
             if (records != null)
             {
diff --git a/src/LogExpert.Tests/ColumnizerTests/CSVColumnizerTest.cs b/src/LogExpert.Tests/ColumnizerTests/CSVColumnizerTest.cs
index ed7a404d..0e03e196 100644
--- a/src/LogExpert.Tests/ColumnizerTests/CSVColumnizerTest.cs
+++ b/src/LogExpert.Tests/ColumnizerTests/CSVColumnizerTest.cs
@@ -62,6 +62,37 @@ private static void SetPrivateField (CsvColumnizer.CsvColumnizer columnizer, str
         field!.SetValue(columnizer, value);
     }
 
+    private const string HEADERLESS_LINE = "2026-09-01 07:09:58.1980; INFO; 2001:7c0:3006:3014::8602:1a1d; ; ; ; zrnsh01 logged in";
+
+    // Valid-CSV mode with "first line contains field names" unchecked
+    private static CsvColumnizer.CsvColumnizer CreateColumnizerWithoutFieldNames ()
+    {
+        CsvColumnizerConfig config = new();
+        config.InitDefaults();
+        config.HasFieldNames = false;
+        config.ConfigureReaderConfiguration();
+
+        CsvColumnizer.CsvColumnizer columnizer = new();
+        SetPrivateField(columnizer, "_config", config);
+        SetPrivateField(columnizer, "_isValidCsv", true);
+        return columnizer;
+    }
+
+    // Valid-CSV mode with ',' as configured delimiter, as if loaded from a saved csvcolumnizer.json
+    private static CsvColumnizer.CsvColumnizer CreateColumnizerWithSavedCommaDelimiter (bool hasFieldNames)
+    {
+        CsvColumnizerConfig config = new();
+        config.InitDefaults();
+        config.DelimiterChar = ",";
+        config.HasFieldNames = hasFieldNames;
+        config.ConfigureReaderConfiguration();
+
+        CsvColumnizer.CsvColumnizer columnizer = new();
+        SetPrivateField(columnizer, "_config", config);
+        SetPrivateField(columnizer, "_isValidCsv", true);
+        return columnizer;
+    }
+
     [Test]
     public void Selected_HasFieldNames_FirstLineNull_FallsBackToCallback ()
     {
@@ -81,6 +112,96 @@ public void Selected_HasFieldNames_FirstLineNull_FallsBackToCallback ()
         Assert.That(columnizer.GetColumnNames(), Is.EqualTo(["name", "age", "city"]));
     }
 
+    [Test]
+    public void Selected_NoFieldNames_ColumnCountFromFirstLine ()
+    {
+        var columnizer = CreateColumnizerWithoutFieldNames();
+        var mockCallback = new Mock<ILogLineMemoryColumnizerCallback>();
+        _ = mockCallback.Setup(c => c.GetLogLineMemory(0)).Returns(new CsvLogLine(HEADERLESS_LINE, 0));
+
+        columnizer.Selected(mockCallback.Object);
+
+        Assert.That(columnizer.GetColumnCount(), Is.EqualTo(7));
+    }
+
+    [Test]
+    public void Selected_NoFieldNames_GeneratesOneBasedColumnNames ()
+    {
+        var columnizer = CreateColumnizerWithoutFieldNames();
+        var mockCallback = new Mock<ILogLineMemoryColumnizerCallback>();
+        _ = mockCallback.Setup(c => c.GetLogLineMemory(0)).Returns(new CsvLogLine(HEADERLESS_LINE, 0));
+
+        columnizer.Selected(mockCallback.Object);
+
+        Assert.That(columnizer.GetColumnNames(), Is.EqualTo(["Column 1", "Column 2", "Column 3", "Column 4", "Column 5", "Column 6", "Column 7"]));
+    }
+
+    [Test]
+    public void SplitLine_NoFieldNames_SplitsIntoFields ()
+    {
+        var columnizer = CreateColumnizerWithoutFieldNames();
+        var line = new CsvLogLine(HEADERLESS_LINE, 0);
+        var mockCallback = new Mock<ILogLineMemoryColumnizerCallback>();
+        _ = mockCallback.Setup(c => c.GetLogLineMemory(0)).Returns(line);
+        columnizer.Selected(mockCallback.Object);
+
+        var result = columnizer.SplitLine(mockCallback.Object, line);
+
+        Assert.That(result.ColumnValues.Select(c => c.FullValue.ToString()), Is.EqualTo(["2026-09-01 07:09:58.1980", " INFO", " 2001:7c0:3006:3014::8602:1a1d", " ", " ", " ", " zrnsh01 logged in"]));
+    }
+
+    [TestCase(true)]
+    [TestCase(false)]
+    public void Selected_FirstLineIsBadData_FallsBackToTextColumn (bool hasFieldNames)
+    {
+        var columnizer = CreateColumnizerWithSavedCommaDelimiter(hasFieldNames);
+        var mockCallback = new Mock<ILogLineMemoryColumnizerCallback>();
+        _ = mockCallback.Setup(c => c.GetLogLineMemory(0)).Returns(new CsvLogLine("\"2021-12-12\"x;\"TRACE\"y", 0));
+
+        columnizer.Selected(mockCallback.Object);
+
+        Assert.That(columnizer.GetColumnNames(), Is.EqualTo(["Text"]));
+    }
+
+    [Test]
+    public void Selected_NoFieldNames_SavedDelimiterDiffersFromFile_DetectsFileDelimiter ()
+    {
+        // Instance that never pre-processed this file, e.g. a clone that loaded ',' from csvcolumnizer.json
+        var columnizer = CreateColumnizerWithSavedCommaDelimiter(false);
+        var line = new CsvLogLine("\"2021-12-12\";\"TRACE\";\"semicolon file \"", 0);
+        var mockCallback = new Mock<ILogLineMemoryColumnizerCallback>();
+        _ = mockCallback.Setup(c => c.GetLogLineMemory(0)).Returns(line);
+
+        columnizer.Selected(mockCallback.Object);
+        var result = columnizer.SplitLine(mockCallback.Object, line);
+
+        Assert.That(columnizer.GetColumnNames(), Is.EqualTo(["Column 1", "Column 2", "Column 3"]));
+        Assert.That(result.ColumnValues.Select(c => c.FullValue.ToString()), Is.EqualTo(["2021-12-12", "TRACE", "semicolon file "]));
+    }
+
+    [Test]
+    public void Selected_HasFieldNames_SavedDelimiterDiffersFromFile_DetectsFileDelimiter ()
+    {
+        var columnizer = CreateColumnizerWithSavedCommaDelimiter(true);
+        SetPrivateField(columnizer, "_firstLine", new CsvLogLine("\"Date\";\"Level\";\"Message\"", 0));
+        var mockCallback = new Mock<ILogLineMemoryColumnizerCallback>();
+
+        columnizer.Selected(mockCallback.Object);
+
+        Assert.That(columnizer.GetColumnNames(), Is.EqualTo(["Date", "Level", "Message"]));
+    }
+
+    [Test]
+    [System.Diagnostics.CodeAnalysis.SuppressMessage("Globalization", "CA1303:Do not pass literals as localized parameters", Justification = "Unit Tests")]
+    public void PreProcessLine_NoFieldNames_KeepsFirstLineVisible ()
+    {
+        var columnizer = CreateColumnizerWithoutFieldNames();
+
+        var result = columnizer.PreProcessLine(HEADERLESS_LINE.AsMemory(), 0, 0);
+
+        Assert.That(result.ToString(), Is.EqualTo(HEADERLESS_LINE));
+    }
+
     [Test]
     public void Selected_HasFieldNames_NoLineAvailable_FallsBackToTextColumn ()
     {