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The breathalyzer companies have tried to keep the code from defendants for a long time, but a courts finally forced them, and it isn't pretty. Ars Technica has a good article on it. I like this part:

The researchers also found that the device doesn't have any built-in sensors to determine if its physical state is consistent at any given time. When the code activates a motor or valve, the report says, it simply assumes that this function has been correctly performed and does not test to make sure. Some diagnostic routines in the code will silently return arbitrary default values upon failure, leading to potentially inaccurate breathalyzer test results. The software will also silently ignore errors in some cases unless there are a large number of consecutive failures.

I expect a high quality of software and hardware engineering to medical device specs if my job, my future are riding on the test results.

The worst part about breath tests is that they are by design readers of an approximation, an assumed 1:2100 ratio of alcohol in the breath to alcohol in the blood. That's only average, not everybody has that. The tests will over- and under-report on certain percentages of the population because of individual physical variables. Yes, your physiology can have you consistently blowing .09 when you're only .07.

Blood test is the only reliable test because it directly measures what they're looking for -- blood alcohol content.