503 Service Unavailable on my site. What's wrong?

Also searched as: 503 service unavailable error · 503 service unavailable meaning

Updated 3 October 2026

Short answer

The server is reachable, but it's saying it can't take the request right now. That's usually one of three things: it's overloaded (out of memory, CPU or database connections), it's in maintenance or partway through a deploy, or the platform has no running copy of your app to send requests to because the app crashed or was switched off.

What's the problem

Visitors see "503 Service Unavailable". It might last a minute during a deploy, come and go at busy times, or stay until someone does something.

Why it happens

  • Maintenance or a deploy. Some setups deliberately return 503 while the app restarts. That should be brief.
  • Overload. When memory, CPU or the pool of database connections runs out, servers turn requests away with a 503 rather than fall over completely. If 503s line up with busy periods, this is it.
  • Nothing is running. On platforms such as Heroku, a crashed app or one that didn't start in time produces a 503. So does an app with its web process scaled down to zero.

How to fix it

  1. Watch how long it lasts. A minute around a deploy is normal. Longer isn't.
  2. Check that at least one copy of your app is running on your host's dashboard. If none is, the logs say why it stopped or failed to start.
  3. Look at memory, CPU and database connection graphs for the times the 503s happened. A line hitting its ceiling is your answer.
  4. If it's load, add capacity (bigger or more instances, a larger database plan) as a stopgap, then find what's using it up.
  5. For planned maintenance, return a 503 with a Retry-After header and a friendly page, so visitors and search engines know it's temporary.

When to call Preventionlabs

If the app had simply stopped, restarting it and fixing the cause in the logs may be all it needs. Call us when 503s keep coming back under ordinary traffic, or the app can't be given more capacity without being rebuilt. A resurrection delivers an app that's running and on infrastructure that adds capacity through configuration, not code changes.

Submit your project for a free assessment

Free assessment. $10,000 AUD flat to get it live, only if we take it on and you go ahead.

Sources

  1. MDN Web Docs: 503 Service Unavailableweb referenceCommon causes are that a server is down for maintenance or overloaded.
  2. MDN Web Docs: 503 Service Unavailableweb referenceIn overload cases, some server-side applications will reject requests with a 503 status when resource thresholds like memory, CPU, or connection pool limits are met.
  3. Heroku: Heroku Error Codes: H10 App crashedhosting platform docsA crashed web dyno or a boot timeout on the web dyno will present this error.
  4. Heroku: Heroku Error Codes: H14 No web dynos running (example log line, status=503)hosting platform docsat=error code=H14 desc="No web processes running" method=GET path="/"
  5. MDN Web Docs: 503 Service Unavailableweb referenceThis response should be used for temporary conditions and the Retry-After HTTP header should contain the estimated time for the recovery of the service, if possible.

Related

502 Bad Gateway: what does it mean, and how do I fix it? The server in front of your app asked your app for the page and got no proper answer. Usually the app has crashed, never started, or is on the wrong port. Heroku says "Application Error". How do I fix it? Heroku's generic page for an app that crashed, didn't start in time, or has nothing running. The code in heroku logs (H10, R10, H14, R14) says which. How do I make my app scale? Make the app stateless so you can run more copies, keep data in managed services, and put capacity behind settings. Then scaling is configuration. My website is down. How do I fix it? Find which layer failed first: the domain, the hosting account, the server, or the app. What the browser shows tells you where to start. A checklist, in order.
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