Excel Macro has randomly stopped working despite working well for several months

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Summary

A long-running Excel macro that ingests data from a web query suddenly failed after six months of stability. The macro is designed to pull specific tables from a URL into a designated worksheet. The failure manifested as a runtime error during the web query refresh process.

The core issue is a hardcoded connection string and a rigid data ingestion logic built on an unstable dependency (Excel’s QueryTables object). The immediate trigger was likely an external change in the data source’s structure or the addition of authentication/challenge layers (like a CAPTCHA or consent banner) that the simple HTTP request could not handle.

Root Cause

The root cause is identified as an external dependency failure combined with brittle code. The macro relies on two highly specific conditions remaining true indefinitely:

  1. The URL endpoint is accessible without authentication: The connection string Connection:="URL;" + myurl assumes the target is a raw data endpoint. If the website administrators placed the data behind a consent wall (GDPR/CCPA), a login, or a WAF challenge, the QueryTable will fail to retrieve the data.
  2. The HTML structure is static: The macro targets WebTables = "2". If the website developers added a new table, reordered them, or changed the table structure (e.g., switching to dynamic JavaScript rendering instead of static HTML), the index “2” no longer points to the intended data set, causing the refresh to fail or import garbage data.

Why This Happens in Real Systems

This scenario is a classic example of “Invisible Infrastructure” breaking. In real-world systems, code often works by scraping or integrating with third-party services that are not under the developer’s control.

  • Unversioned APIs: Websites rarely version their HTML tables. A layout change is treated as a non-breaking change by the site owner, but it is catastrophic for a scraper.
  • Security Headers: Modern browsers handle security redirects and cookies silently. VBA’s QueryTable is less sophisticated. If the site introduced a redirect (e.g., HTTP to HTTPS) or a cookie consent requirement, the VBA client might reject the stream.
  • Data Volume: The macro clears the sheet (UsedRange.ClearContents) and then pulls data. If the external source suddenly provided significantly more data than the Range("A1:A250") assumption, it could cause performance issues or display problems, though the specific error points to the refresh itself.

Real-World Impact

  • Business Process Stoppage: The user relies on this data for daily work. The “randomly stopped working” description implies a loss of productivity and potential manual re-entry of data.
  • Debugging Blindness: The user cannot debug it because they are looking for changes in their own code, which hasn’t changed. They are missing the context that the environment changed.
  • Loss of Trust: Automated tools that fail silently or cryptically cause users to abandon them or revert to manual processes.

Example or Code

Below is the reconstructed logic causing the failure. The critical point of failure is the .Refresh method inside the With block, which relies entirely on the stability of myurl.

Sub Hour_1()
    Dim myurl As String
    ' If A39 is empty, malformed, or points to a new page structure, the macro fails.
    myurl = Worksheets("Config").Range("A39").Value

    Sheets("H1").Visible = True
    Sheets("H1").Select

    ' This clears data. If the refresh fails, the sheet remains empty.
    Sheets("H1").UsedRange.ClearContents

    With ActiveSheet.QueryTables.Add(Connection:="URL;" + myurl, Destination:=Range("$A$1"))
        .Name = "287%2C321..."
        .FieldNames = True
        .RowNumbers = False
        .FillAdjacentFormulas = False
        .PreserveFormatting = True
        .RefreshOnFileOpen = False
        .BackgroundQuery = True
        .RefreshStyle = xlInsertDeleteCells
        .SavePassword = False
        .SaveData = True
        .AdjustColumnWidth = False
        .RefreshPeriod = 0
        .WebSelectionType = xlSpecifiedTables
        .WebFormatting = xlWebFormattingNone
        ' CRITICAL: This assumes the data is always in the 2nd HTML table found on the page.
        .WebTables = "2"
        .WebPreFormattedTextToColumns = True
        .WebConsecutiveDelimitersAsOne = False
        .WebSingleBlockTextImport = False
        .WebDisableDateRecognition = False
        .WebDisableRedirections = False

        ' This is likely where the error occurs. The connection is valid, 
        ' but the content is rejected or the structure is invalid.
        .Refresh BackgroundQuery:=False

        ' The macro attempts to format immediately after.
        Range("A1:A250").TextToColumns _
            ConsecutiveDelimiter:=True, _
            Comma:=True
        Application.DisplayAlerts = False
    End With
End Sub

How Senior Engineers Fix It

Senior engineers solve this by decoupling the code from the volatile environment and implementing defensive programming.

  1. Use Robust Data Protocols: Replace QueryTables with MSXML2.ServerXMLHTTP to fetch the raw HTML or JSON. This allows for better error handling, header inspection, and debugging of the actual HTTP response (e.g., seeing a 403 Forbidden or 200 OK with a challenge page).
  2. Implement Logic to Handle HTML Changes: Instead of relying on WebTables = "2", write logic to search the raw text for specific identifiers or headers that define the data set, making it resilient to table reordering.
  3. Standardize Data Input: Ideally, stop scraping HTML entirely. If the data provider offers a CSV, API, or RSS feed, use that. It is structured and versioned.
  4. Add Error Handling: Wrap the .Refresh call in an On Error block to capture the specific error description (e.g., “302 Moved Permanently” or “403 Forbidden”) so the user knows why it failed, rather than just that it failed.

Why Juniors Miss It

Junior developers often focus on the syntax of the code rather than the context of the execution.

  • “It worked yesterday”: They assume code is immutable and permanent. They do not anticipate that an external URL (which they treat as a constant) is actually a variable.
  • Visual Reliance: They rely on the Excel UI to debug (e.g., stepping through code). If the web query returns a “Accept Cookies” prompt disguised as a table, the code might run but import the wrong thing (or nothing), leading to confusion.
  • Lack of Network Awareness: Juniors may not understand that Excel VBA acts as a network client. They don’t check for network issues, timeouts, or changes in the website’s source code, looking instead for bugs in their VBA logic.