A well-designed 404 Not Found page is crucial for improving user experience and retaining customers who land on unavailable pages. OpenCart allows you to customize this page to align with your branding and provide useful navigation options.
Mostly, the 404 page looks like below in Opencart:

Steps to Customize the 404 Page in OpenCart
We used to need the custom module or extensions for these kinds of functionalities, but with the introduction of Language Editor in Opencart, it is easier. If you are looking to just edit the "The page you requested cannot be found!", then you can easily do it by the language editor of Opencart. Go to the admin section >> Design >> Language Editor and click the Add button
Enter the following details:

Now the 404 will look like below:

Read more: Language Editor in OpenCart
Example 404 page:
Here is one design and code example to show the different titles for Category not found, product not found, and other not found.
Design as per category, product, and other page:



Add the following code to the language editor:
Here is one language editor:

The JavaScript in the code finds the route and shows an error message as per the product, category another page. If you need a similar design, then you need to select a different language in the Language editor and enter a different value as per language.
Add Google Analytics Tracking
To monitor the 404 errors on your site:
- Use Google Analytics to track 404 errors.
Set up a custom report for pages with the "404 Not Found" response. - Add a tracking script to the 404 page to log user behavior. Example:
Best Practices for a 404 Page
- Provide Helpful Suggestions: Include links to your homepage, categories, or popular products.
- Apologize for the Inconvenience: Acknowledge the error with a friendly, empathetic text.
- Use Humor or Branding: Lighten the mood with creative messaging that reflects your brand’s personality.
- Include a Call-to-Action: Guide users back to the shopping experience with CTAs like "Continue Shopping" or "Search Our Store."
Conclusion
Customizing the 404 Not Found page in OpenCart is an effective way to improve user experience and reduce bounce rates. By adding helpful navigation tools, engaging content, and personalized branding, you can turn an error page into an opportunity to retain potential customers and guide them back into your store.