There are many variables that I would like to know about your situation, including did you blow into a breath device (PAS or EPAS), do you have a medical condition which results in the contents of your stomach coming into your mouth, or any dental condition which could result in mouth alcohol, what are the facts of the calibration of the breath device, etc. I would consult with an experienced DUI attorney because there are truly many different things that can result in a successful defense.
With a .13 BAC - the District Attorney would only offer a wet reckless if the case had other serious issues. If the case has other serious issues - its worth "wasting money" on one of us!..
That depends on the lawyer you hire, as well as the facts of the case, and what can be proven by the District Attorney. Make sure you do your homework on who you're hiring. Some Attorneys just go in and do the "best" they can without performing the necessary investigation/research. On that end, you should okay searching the CA DUI Lawyers Association and choosing a few to meet with from there.
Second, the investigation is extremely necessary because it will bring out any favorable evidence. Including CHP dashcam video that will be available about 99% of the time. Often, this video will record voice as well (although it may not be the best quality).
As for BAC level, it depends on the case details and how they work in your favor. I've had .19 result in a "wet". I've had .10 BACs result in a "dry". You just never know for certain until the investigation is done. You could hire the right lawyer, but nothing in the investigation ends up helping you. Its too hard to predict. But you will always be better off hiring a lawyer, then simply pleading guilty, without having someone with the necessary experience to review all of the evidence.
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Unless there are some serious issues with your case, you are not going to get a wet reckless offer from the DA with a .13. As for wasting money on a lawyer, if your plead to a 23152(b), which is what the DA's are insisting on, you'll be on informal probation for 3 years, the DUI will remain on your record for 10 years, which means higher insurance premiums, you'll be paying over $2,100 in fines and fees, you'll have to attend a 3 month drivers education course, you may have to install a breathalyzer in any vehicle you own or drive and you'll have to do at least one day of work project. Many attorney offers a free or low cost consultation, so I doubt it would be a waste of time or money. If nothing else, if you fall below a certain income limit, you may be eligible for the public defender but that means making less than about a $1,000 a month.
Any potential reduction depends on two major factors - the jurisdiction where the case will be heard and the particular facts of your case. Some courts officer a wet reckless when the BAC is .11 or less - some clever arguments or exigent circumstances will convince the prosecutor and or the court to give you a deal. Your attorney should fight for you.