Why did my GPX file become a course instead of an activity?

That is Garmin Connect working as designed. GPX describes tracks and routes, not workouts, so uploads land in the Courses section. To get an entry in your activities feed with distance and time, upload a FIT or TCX file, or convert your GPX to FIT first.

What is a CRC error on a FIT file?

FIT files end with a checksum computed when the file was written. If the bytes changed after the fact — a cut-off transfer, a dying storage chip — the checksum no longer matches and strict parsers refuse the file. A repair tool can rebuild the header and checksum around the intact portion.

The file works in Strava but not Garmin Connect. Why?

Different strictness. Strava's parser often salvages the readable portion of a marginal file, while Garmin Connect rejects the whole upload when structure or timestamps fail validation. Repairing the file usually satisfies both.

My upload finished but the activity never appeared. Where did it go?

Check the Courses list and the duplicate filter. GPX files become courses, and Garmin Connect silently discards an upload whose timestamps match an existing activity rather than creating a second copy.

Why Won't Garmin Connect Accept My File? Five Real Reasons

You have a file. Garmin Connect has an error message — or worse, a silent no-op. In practice, almost every rejected upload comes down to five causes. Run them in order; the list goes from most to least common.

1. Wrong file type for the destination

Garmin Connect accepts FIT, GPX and TCX, but it treats them differently. FIT and TCX uploads become activities. GPX uploads become courses — they land in a different section, without distance or elapsed time in your feed, and people read that as a rejection.

If your goal is an activity entry and you are holding a GPX, convert it first: GPX to FIT produces a device-style activity file that uploads normally. Going the other way, FIT to GPX gives you the universal track format for mapping tools — just don't expect it back in your activities feed.

2. A truncated transfer

FIT files are written sequentially: header first, records in order, checksum at the very end. If a copy from the device gets cut short — the cable unplugged a moment too early, a Bluetooth sync interrupted, a USB hub that hiccuped — you get a file that ends mid-record and fails immediately.

The tell: the file is noticeably smaller than your other recordings of similar length, or a parser reports an unexpected end of file.

Two fixes, in order of preference. First, re-export from the device: the original recording often still lives on the watch or head unit, and a fresh copy comes out clean. If the device has already overwritten it, a repair pass can rebuild the trailer around everything that did transfer — try the FIT repair tool.

3. A checksum (CRC) failure

Every valid FIT file ends with a CRC checksum covering its contents. Corruption that changes bytes in place — failing flash storage, a bad sector, an interrupted re-save by some other tool — breaks the match, and Garmin Connect declines the whole file even when 99% of the recording is fine.

This is the classic case for repair rather than re-export: the tool walks the file message by message, discards whatever is genuinely unreadable, rebuilds the header and checksum around the intact data, and hands you a valid file containing everything that survived. What was destroyed was never recoverable by anyone; the point is to stop the damaged tail from taking the healthy 95% down with it. (The full repair walkthrough.)

4. Timestamps that do not add up

Platforms validate the timeline before accepting an activity. Common trip-wires:

A clock reset after a dead battery, leaving the device recording in 2014 or in the future.
Timezones applied twice by a conversion tool, shifting start times by hours.
Overlapping or non-monotonic records from a stitched-together or edited file.

Garmin Connect's duplicate and sanity filters reject these on arrival. If the timestamps came out of a merge or conversion, redo it — merging FIT files orders records chronologically and rebuilds the session timeline, which is exactly what the validator wants to see. If the device clock itself was wrong, the cleanest fix is correcting the times before upload rather than fighting the platform.

5. Duplicate detection

You uploaded the same recording twice — perhaps once from the device sync and once by hand. Garmin Connect frequently accepts the transfer and then quietly drops the second copy, because its timestamps and distance match an existing activity. Nothing is broken; nothing needs fixing. If you want the second copy (say, a merged or repaired version), delete the original activity first, then upload.

A quick diagnostic order

Check the extension and where the file landed — GPX goes to Courses. (Convert if you wanted an activity.)
Compare the file size against similar recordings — suspiciously small means truncation.
Run it through the repair tool — it reports what it found instead of guessing.
Sanity-check the start time in a viewer before re-uploading.
Delete the near-duplicate activity, then upload the replacement.

That sequence resolves nearly every rejected upload we see — and when it doesn't, the repair tool's report tells you exactly which of the five walls you hit.