How DebugView works
Ordinary GA4 reports are processed and aggregated, which makes them useless for checking whether a tag you changed a minute ago actually fires. DebugView solves that by carving out one device. When a visit is marked as being in debug mode, GA4 streams its events into a separate screen almost as they happen, in order, with every parameter attached.
A device gets marked in one of a few ways. Opening Google Tag Manager’s preview mode flags the session automatically. Google’s own Chrome debugger extension does the same. If you are working with a developer, the flag can also be sent directly with the event as a debug parameter, which is how app and server-side setups usually do it.
What you then see is a timeline of events with a click-through into each one. Select an event and you get its parameters, the user properties in force at that moment, and the value of each. That is the level of detail you need to confirm that a purchase event carried a value and a currency rather than firing empty.
Why DebugView matters
Almost every serious tracking fault I find is a tag that fires but carries the wrong payload: a lead event with no form name, a purchase with a value of zero, a parameter spelled one way in the tag and another way in the report. None of that is visible in a standard report until days of data have been ruined.
It also stops the guesswork that follows a website change. When a new theme, a new form plugin or a new checkout goes live, a couple of minutes in DebugView answers the question that would otherwise take a week of waiting to answer badly.
Common mistakes with DebugView
The first is treating an empty screen as proof that nothing is being collected. DebugView only shows the flagged device, so if the flag has dropped — preview mode closed, a redirect to a different domain, a page opened in another browser — the screen goes quiet while collection carries on perfectly well.
The second is checking that an event appears and stopping there. The event name is the easy half; the parameters are where the money is. The third is testing only the happy path. Test the form that fails validation, the visitor who declines cookies and the phone browser, because those are the journeys real customers take.
How to act on it
Make it a habit rather than a rescue tool. Before any tracking change is called finished, walk the site as a customer would with debug mode on, trigger each important event, open it and read its parameters against what your reports expect.
Test on a phone as well as a desktop, which matters more in markets like Nepal where most traffic is mobile. Then confirm the same events appear in the standard reports the next day, because DebugView proves collection, not processing. If you inherited an account and have no idea what fires, a structured tracking audit is faster than clicking around hoping to spot the gap.