Plesk Panel 503 Service Unavailable Error Definitive Solution

Why Does Plesk 503 Service Unavailable Happen?

The 503 Service Unavailable error, which you may encounter suddenly especially if you are using a Plesk Panel running on a Windows Server infrastructure or hosting a highly resource-consuming site on Linux Plesk, indicates that the Application Pool behind the site has crashed or is too busy to respond to the service.

Pool Crash on Windows Servers (IIS)

In the Windows hosting infrastructure, each website runs in its own "Application Pool" on IIS. If your site's software has an infinite loop or exceeds the RAM limit, IIS stops this pool for security purposes. To solve it:

1. Log in to the Plesk Panel.
2. Click on the site giving the error and open the "Dedicated IIS Application Pool for Website" option from the menu.
3. Check the status of the pool. If it appears as "Stopped", you can bring your site back by pressing the Start button.
4. To prevent the problem from reoccurring, try to increase the performance (Memory Limit, CPU Limit) settings on the same screen.

Lack of Limits on Linux Servers (PHP-FPM)

In Plesk installations on Linux, the main reason for this error is reaching the limits of the PHP-FPM pool allocated to the site (pm.max_children limit). The solution is very simple:

1. Enter your site's section in Plesk.
2. Click on the "PHP Settings" menu.
3. Expand the "Performance and security settings" tab at the bottom.
4. Increase the pm.max_children value from, for example, "5" to "20" and save.

Increasing PHP settings too much can consume the overall RAM amount of your server and cause other sites to crash as well. To say goodbye to such resource and optimization problems, you can review our unshared virtual server (VDS) packages with high CPU and RAM power and enjoy the performance.