If you bought the monitor directly from Dell, you received an electronic and paper invoice with your Dell order number and Dell customer number. Technical Support for your country should be able to look those up to see when you purchased the monitor and the duration of the warranty period.If you bought the monitor from a reseller, the express service code and tag may have originated in another region of the world which is why the Dell website in your region cannot see it.
* Read though this* Then go here and run all of the test. Be prepared to show those results to Dell Technical Support for your country
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