I am sorry to hear that you are going through this. Most courts are going to understanding with situations like yours. I do not know how the particular machine works but your 4 months of good readings are going to go a long way to bolstering your credibility. I hope that things work out for you and wish you the best.
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Every type of device has some built in margin error, but they are supposed to be free of false readings. It may be worth your while to contact the manufacturer to see what type of activities substances might cause the device to give a false indication.