In order to compel a breathalyzer, the officer must have reasonable suspicion that you are intoxicated. Swerving a little bit might not reach that threshold, since maybe they didn’t see the obstacle you swerved to avoid, or there’s a mechanical issue. They can pull you over for the swerving, which is enough to compel a field sobriety test, especially if they smell alcohol. If you fail it, they can compel you to either take the breathalyzer, or compel a blood sample drawn. Depending on the jurisdiction, you can refuse the breathalyzer, but it may be an automatic suspension of license.
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