This certainly can be a serious issue when you are on a deferred prosecution because total abstinence is one of the conditions. If your interlock company does report the result to probation, the court will normally set a probation/review hearing to address the matter. If you deny the positive UA, the court can request evidence be presented, usually in the form of documents or testimony supporting the UA. You would also have an opportunity to contest the charge as well. The Court will then make a determination whether the claim has been proven or not and if so, can impose sanctions. One of those is revocation of your deferred prosecution. The best thing you can do is discuss this matter with your attorney. If you do not have one, hire a competent attorney experienced in DUI law immediately. The stakes could be pretty high.
I am reluctant to offer any advice without more information but I think you have to act fairly quickly. Certain U/As can test for alcohol/drug consumption for several days and may rebut the insinuation from a failed ignition interlock. U/As can be easily doctored and I don't know how much weight your particular judge would give a U/A because of this.
Blood tests are probably more relaible than U/As and cost about the same.
Some hair follicle tests can now check for up to 3 months of alcohol consumption although most hair follicle tests only check for drugs other than alcohol. I think if you are looking at strike 2 on a D.P. getting some reliable evidence before the court you are clean is very important which is why some sort of negative drug test sould be very useful in your case.
This can be a very serious issue. Your IID is very sensitive. They should have told you at the time of installation all of the various products that could activate a failed test. With a probation violation already on your case file it is important that your attorney, if you have one, to be able to argue on your behalf the reason for the failed test. There could be many reasons for it. If you do not have an attorney I would suggest obtaining one. As one of my colleagues said before, find one that is experienced in DUI law.