If you are arrested on suspicion of driving under the influence, the police will probably give you a breath test. The goal of this test is to measure your blood alcohol concentration. They are essentially checking to see if alcohol is present in your system and especially if it is over the legal limit, which is generally 0.08% for most drivers.
That being said, regular maintenance and calibration are necessary with breath tests devices. If this has not been done, or if the test was poorly calibrated, it can call the results into question. In other words, you may have failed a breath test that showed you were over the legal limit, but if you can demonstrate that routine calibration procedures were not followed, there is no real way to determine if that evidence is accurate or not.
Unreliable devices
A similar issue is that courts have found that some breath test devices are unreliable in general. As such, police departments usually have an approved make and model that they are supposed to use. If an aftermarket device was used to administer your test, the evidence may not hold up in court.
Finally, even if the device itself works properly and is an approved model, you may want to question if the officer made mistakes while administering that test. Perhaps the police department never gave that officer any formal training, so it is not clear if they even knew how to give out the test appropriately.
All of these issues could have a significant impact on the evidence in your case. It is important to consider them carefully while looking into your legal defense options.