VAST Error 603: Unable to fetch companion resource
The companion's image/iframe/HTML asset couldn't be downloaded — dead URL, block, or timeout.
The companion matched a slot but its resource failed to load. Identical failure classes to 502, applied to companion assets: 404s from expired campaigns, ad-blocker interception, mixed content, slow hosts.
Why error 603 happens
Expired or moved creative asset
The banner image 404s after campaign assets were cleaned up.
Ad blocker interception
Companion banners are display ads to a blocker — among the most-blocked resources in the ecosystem.
How to fix VAST error 603
- 1
Fetch the companion URL directly
The HTTP status separates missing assets from blocked ones.
- 2
Serve from HTTPS CDN with correct headers
Removes mixed-content and latency failure modes.
Seeing error 603 on a live tag?
Paste the tag URL or XML — the validator resolves the full wrapper chain, validates the schema, downloads every media file, and shows exactly which stage fails.
Validate your VAST tagVAST error 603 — frequently asked questions
How much companion loss is from ad blockers?+
On desktop web, a substantial share — companion assets match display-ad blocklists precisely. Measure by comparing companion fetch failures on blocked vs unblocked sessions.