Why Does My Application Pool Keep Stopping? (App Pool Auto-Disabled / 503 Service Unavailable) Print

What's happening

If your website's application pool keeps stopping or you have to keep restarting it (and the site intermittently shows 503 Service Unavailable), it is almost always one of these — in order of likelihood:

• Your application is running out of memory, or has a memory leak.
• Your application code is throwing errors — unhandled exceptions, a startup failure, or a bug that crashes the worker process.

When the worker process crashes repeatedly, IIS Rapid-Fail Protection automatically disables the pool to protect the server — which is why it "keeps stopping."

Important: a one-off "it's running fine right now" check doesn't mean the problem is gone — the pool simply happens to be up between crashes. The pattern is what matters.

What you can do yourself

1. Look at your application first (most common cause). Check for memory leaks, unhandled exceptions, or startup errors. For ASP.NET Core, set stdoutLogEnabled="true" in your web.config to capture the crash detail in your logs folder, then review and fix the underlying error.
2. Need more memory? Upgrading your plan is usually the best value. Each tier up includes more application-pool RAM plus more websites, storage, bandwidth and databases — typically at a lower cost per GB than buying RAM separately. If you only need a small top-up and want to stay on your current plan, additional App Pool RAM add-ons are available in your Client Area under Services > your hosting plan > Addons (you can add more than one). For a meaningful increase, ask us and we'll help you compare a top-up vs. the next plan up.
3. Check your dedicated pool — only if you turned it off. Dedicated, isolated application pools are on by default, so your site is only on the (busier, less stable) shared pool if you previously switched yours off. If that's the case, in Plesk go to Websites & Domains > (your domain) > Dedicated IIS Application Pool and switch it back on (only you can change this). You can also recycle the pool from the same screen.

What our team handles (server / IIS side)

A couple of settings live at the IIS level on the application pool itself and are not available in Plesk, so our team takes care of them. If your pool keeps being auto-disabled, open a support ticket and we will:

• Review the Windows Event Log to confirm exactly why the pool is failing, and
• Relax or disable Rapid-Fail Protection on your pool so it stops auto-disabling while you address the underlying app issue.

Please don't spend time looking for these options in Plesk — they aren't there. Just tell us the symptom and your domain, and we'll apply the server-side fix.

When to contact us

Reach out if the pool keeps stopping after you've reviewed your application (and you haven't turned your dedicated pool off), or if you'd like us to check the server logs and adjust the IIS-level settings. Open a ticket from your Client Area with your domain name and roughly when the stops happen, and we'll take it from there.