How to Read Status Codes
Status codes are defined by the HTTP standard (currently RFC 9110). The first digit sets the category: 2xx means success, 3xx a redirect, 4xx a problem with the request (typically a page that does not exist) and 5xx a server failure. For an advertiser only one thing matters: only a 2xx response (after any redirects) means the visitor actually saw content.
200 OK
The page loaded. This is what you want to see for every destination URL in every campaign. Careful though: a 200 says nothing about what loaded. An empty page, or a page saying "product not found", can also return 200 (more on that under soft 404 below).
301 / 302 / 308
Redirects. 301 and 308 are permanent, 302 is temporary. A redirect itself is not an error, but each one in a chain adds load time and another place for things to break: redirect chains sometimes strip UTM parameters or end up in a loop. The ideal for a paid campaign is linking straight to the final URL.
404 Not Found
The page does not exist. The internet's most famous error code and the most common reason for wasted ad budget: the ad runs, clicks get billed, and visitors stare at an error page. The conversion rate of a visit that ends on a 404 is zero by definition.
410 Gone
The page existed and was deliberately removed. To a user it looks the same as a 404. To search engines it is a stronger signal to drop the page from the index. If a 410 shows up on the destination URL of a running ad, someone deleted the page and forgot about the campaign pointing to it.
Soft 404
A treacherous special case: the server returns 200, but the page content says "not found", shows an empty result, or bounces to the homepage. Google officially describes this as a soft 404. For ads it is doubly dangerous, because a basic status code check will not catch it; only checking the actual page content will.
500 / 502 / 503 / 504
Server errors. 500 is a generic internal failure, 502 and 504 usually mean trouble between a proxy and the application, 503 means overload or maintenance. From a campaign's point of view they are all the same: the visitor got no content. Unlike a 404, 5xx errors tend to be transient, which makes knowing when they started and whether they persist even more important.
The only response category where an ad click can turn into a customer.
What to Do With This as an Advertiser
- When launching a campaign, open the exact destination URL (including UTM parameters) in an incognito window and watch where you end up.
- Minimize redirect chains: use the final HTTPS version of the URL in your ad, with the correct www or non-www form.
- Check destination URLs continuously, not just at launch. Pages break while campaigns run: during site edits, migrations and certificate expiries. 404Watcher runs these checks automatically every hour and distinguishes 404s, 5xx and SSL errors.
Key Takeaways
- Only a 2xx response means the visitor saw content. Everything else is a loss for the campaign.
- A soft 404 masquerades as success: the server returns 200 but the content is missing. Only a content check catches it.
- 5xx errors are transient, which makes them sneaky: without continuous monitoring you will never learn about an overnight outage.