Select Page

How to resolve IIS “HTTP Error 503. The service is unavailable.” error in hosting servers

In our role as Web support specialists, we resolve and prevent service errors in both Linux and Windows servers. Recently we were contacted by a Windows server owner who was getting “HTTP Error 503. The service is unavailable” in his websites.

Upon examining the EventViewer logs, we could see the error “A process serving application pool ‘plesk(default)(4.0)(pool)’ suffered a fatal communication error with the Windows Process Activation Service”.

See how we help web hosting companies

Today we’ll see what causes IIS “HTTP Error 503. The service is unavailable.” error and how we resolved it.


What causes IIS “HTTP Error 503. The service is unavailable.” error

Our Windows server experts noted that all the domains in the server were using the same application pool [ie. ‘plesk(default)(4.0)(pool)’]. In our experience handling such errors, the following causes have been noted for this error.

  1. Application Pool of the corresponding Wep Application is Stopped – In an IIS server, the domains are assigned to application pools, some dedicated and some shared. If the associated application pool for a domain is in stopped or disabled state, it would cause the website to show “HTTP Error 503. The service is unavailable.” message.
  2. Any misconfiguration in the application pool or site settings, such as spawning applications in 32-bit mode, that can tamper with its proper functioning, can cause error in the site. Improper IIS configuration have noted to cause the worker process to fail. Process crashes also happen due to incorrect application logic. In some cases, the web server protocol may not be enabled in the site settings, and the site can show up ‘HTTP Error 503.’
  3. Issues related to the user Identity of application Pool – An application pool is associated with a user identity, which allows you to run that application pool under a unique account. This user account can get locked at times, can have its password expired, or may not have adequate privileges to run the application pool. Any of these issues can tamper with the website functioning.
  4. Application pool running out of RAM or other server resources, causing it to crash, often lead to ‘HTTP Error 503.’
  5. Server migrations have also seen to have caused such errors in the website, if the IIS configuration settings of two servers are not compatible with each other, which can affect domain performance.

[ You don’t have to lose your sleep over server errors. Our expert server support specialists monitor & maintain your servers 24/7/365 and keep them rock solid. ]


How to resolve IIS “HTTP Error 503. The service is unavailable.” error

To pinpoint the actual cause of the 503 error, we examine the System event logs, which gives a hint towards what is the reason for the error. Based on that, we follow various debugging methods to resolve it.

If the error is due to a stopped application pool, then usually starting it would resolve the issue. From the IIS Manager -> Application Pools node, right click on the DefaultAppPool to check the status.

On noticing that the service is stopped, we start it. If the service is running, we try restarting it and the error would be gone. But in most cases, the fix may not be as simple as that.

In cases where the application pool crashes frequently, restarting it all the time is not the right way to fix it. We monitor the server and debug further into what is causing the application pool to crash.

For websites that are resource-intense, it is always better to assign a dedicated application pool for that domain. Another aspect we check and fix the settings of application pool and site, such as enabling the relevant protocol.

One of the key issues many fail to notice, is the user identity settings. We check the account settings and password of the identity and confirm that it is working fine. If mismatch noted, we would update the settings.

However, in this specific scenario, the issue was due to inadequate permissions of the application to run, which caused the error. By reconfiguring the application pool to run with a higher privileged identity, we were able to resolve the issue.

To avoid resource crunch leading to application crashes, we also optimized the IIS server and could see improvement in the capacity to handle connections with lesser CPU usage.


Use your time to build your business. We’ll take care of your servers. Hire Our server experts to resolve and prevent server issues. ]


At Bobcares, our 24/7 Web hosting technicians constantly monitor all the services in the server and proactively audit the server for any errors or corruption in them.

With our systematic debugging approach for service or other software errors, we have been able to provide an exciting support experience to the customers.

If you would like to know how to avoid downtime for your customers due to errors or other service failures, we would be happy to talk to you.



Never again lose customers to poor server speed! Let us help you.

Our server experts will monitor & maintain your server 24/7 so that it remains lightning fast and secure./p>


Submit a Comment

Your email address will not be published. Required fields are marked *

Bobcares is a server management company that helps businesses deliver uninterrupted and secure online services. Our engineers manage close to 51,500 servers that include virtualized servers, cloud infrastructure, physical server clusters, and more.

Privacy Preference Center


    Necessary cookies help make a website usable by enabling basic functions like page navigation and access to secure areas of the website. The website cannot function properly without these cookies.

    PHPSESSID - Preserves user session state across page requests.

    gdpr[consent_types] - Used to store user consents.

    gdpr[allowed_cookies] - Used to store user allowed cookies.

    PHPSESSID, gdpr[consent_types], gdpr[allowed_cookies]


    Statistic cookies help website owners to understand how visitors interact with websites by collecting and reporting information anonymously.

    _ga - Preserves user session state across page requests.

    _gat - Used by Google Analytics to throttle request rate

    _gid - Registers a unique ID that is used to generate statistical data on how you use the website.

    smartlookCookie - Used to collect user device and location information of the site visitors to improve the websites User Experience.

    _ga, _gat, _gid
    _ga, _gat, _gid


    Marketing cookies are used to track visitors across websites. The intention is to display ads that are relevant and engaging for the individual user and thereby more valuable for publishers and third party advertisers.

    IDE - Used by Google DoubleClick to register and report the website user's actions after viewing or clicking one of the advertiser's ads with the purpose of measuring the efficacy of an ad and to present targeted ads to the user.

    test_cookie - Used to check if the user's browser supports cookies.

    1P_JAR - Google cookie. These cookies are used to collect website statistics and track conversion rates.

    NID - Registers a unique ID that identifies a returning user's device. The ID is used for serving ads that are most relevant to the user.

    DV - Google ad personalisation

    IDE, test_cookie, 1P_JAR, NID, DV, NID
    IDE, test_cookie
    1P_JAR, NID, DV