You can be found guilty of DUI even if you blow a 0.0 on breathalyzer.  A breathalyzer only tests for alcohol on your breath.  It does not test for other types of substances that can lead to intoxication.  Texas law does not limit DUI offenses to those only as a result of alcohol.  Whether you are intoxicated because of a legal prescription, Nyquil, or some other substance, as long as you did not have the normal use of your mental faculties, then you can be found guilty of DUI.  For example, if the officer smelled marijuana in your car, made notes about the condition of your eyes being consistent with use of marijuana, and if you mentioned that you had just smoked a joint or two--- that could be enough evidence for some juries to convict you of DUI.  The breathalyzer is only one type of evidence they can use against you.  If the officer, however, is alleging that you were intoxicated because of alcohol, then you have a better chance of getting your case dismissed--- since the type of equipment used to gather evidence against you was specific to alcohol.  Even though the judge is suggesting that you plea not guilty, you may still want to visit with an attorney that practices criminal law in your area and review the offense report of your arrest.  The judge may be attempting to give you helpful advice, not realizing that other factors may be at play.  Knowing how you are going to be charged and what evidence they intend to offer will be the controlling factor on if and how you can be charged with DUI with a 0.0 on the breathalyzer.