Google Geocoding API - REQUEST_DENIED

I found that in my case, calling to the service without secure protocol (meaning: http), after adding the key=API_KEY, cause this issue. Changing to https solved it.

Until the end of 2014, a common source of this error was omitting the mandatory sensor parameter from the request, as below. However since then this is no longer required:

The sensor Parameter

The Google Maps API previously required that you include the sensor parameter to indicate whether your application used a sensor to determine the user's location. This parameter is no longer required.

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Did you specify the sensor parameter on the request?

"REQUEST_DENIED" indicates that your request was denied, generally because of lack of a sensor parameter.

sensor (required) — Indicates whether or not the geocoding request comes from a device with a location sensor. This value must be either true or false

Remove the API key parameter and its value.

eg. https://maps.googleapis.com/maps/api/geocode/json?address=[YOUR ADDRESS]&sensor=true