It's not quite that easy but you're on the right track prior DUI conviction
from other states are not necessarily prior conviction that can be used in
California but there's more to it it depends what you actually plead to
The District Attorney's office is likely to file this a second offense and allege the out of state prior. Whether the prior actually constitutes a prior in California depends on the wording of the law to which you plead in Illinois and the contents of all the documents pertaining to the plea in Illinois. This needs to be carefully examined by a criminal defense professional. It may be subject to a contested hearing in the new case or could be resolved by agreement with the prosecutor. If you obtain a reduced charge in California, the prior is not applicable. If you are convicted in California, the potential prior will need to be addressed. The current state of your residence and driver license is an issue to be considered also. Many states have laws that make being in physical control of a vehicle while under the influence equivalent to driving under the influence. Violation of these laws may make it hard to successfully establish a prior in California. The circumstances that you describe could potentially result in a conviction, depending on the circumstances, in a jurisdiction that requires driving or only being in control. Driving could be established by circumstantial evidence including the remoteness of the location, being the sole person about the location, and other evidence of alcohol consumption and driving including your own statements to an officer. Many attorneys are not aware of all the issues, so you need to get a qualified professional if you want the best chance of success. There is a lot at stake for you with respect to the potential consequences.
I agree that if the Prosecution knows about it they will simply file it as a second DUI. Ultimately, it is their burden to prove that the Illinois prior satisfies the requirement to be able to treat it as a prior in California. Ventura can be a strict jurisdiction. I would contact a knowledgeable DUI attorney to assist you.
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What matters is how the case was settled in Illinois whether it can be used as a prior conviction. In CA the DMV will want to use all priors they can find. Even if the DA's office in Ventura doesn't pick up the conviction as prior you may have significant and long term issues trying to get your license to drive back.
make sure you consult with an attorney that is familiar with the DMV ramifications. You should obtain a "long form" printout of your record from the CA DMV. This way your potential attorney can decide the best course of action. "Priors" that are not charged can cause big headaches later.
Ventura will probably pick up your first DUI. They are not going retry your first case. Ventura is very tough on DUI's. You might be able to set aside the first conviction-but it will be a challenge.
In order to be considered a prior offense in CA, the offense in Ill. must be substantially similar to what makes it a crime in CA. (CVC 23626.) Your description of the Ill. event suggests proof of the driving element might be lacking in CA. However, the substantially similar test is based upon the language of the respective statutes (Ill. v CA) and not the underlying facts of your specific case. The validity of the prior is based upon the certified record of conviction paper work from the original court and a comparison of the respective DUI laws. If Ill.'s statutory "driving" element of a DUI is not a stringent as CA's "driving" requirement, then the prior cannot be used against you. (There are several states for which this is true - their requirement of "driving" is not as strict as CA's.) Court's do not re-litigate prior offenses. Thus, the Court is not going to look at the underlying facts of the prior (i.e. the police report.) It is only going to look at the certified documents the prosecutor files when he/she seeks to prove the prior.
However, given the age of the prior (2006), it's out-of-state origin, and the facts you relate, a good DUI attorney may be successful in negotiating a disposition with the DA that does not include the prior conviction allegation. Ventura is a very strict DUI prosecution jurisdiction. They've had a "no plea bargain" rule for over 20 years now, but that doesn't mean they won't negotiate with regard to the prior conviction.
Your best start is a good DUI lawyer, a review of the Ill. DUI law and its interpretation, and a review of the certified documents and police report from Ill.
Just being "outside of the car and the keys out of the ignition" does not mean it would not be a DUI in CA. CA has plenty of "no driving/movement of the vehicle observed" DUIs on the books. In CA "driving" and "movement of the vehicle" can be proved by circumstantial evidence as opposed to direct evidence. But, driving and movement must still be proven in CA. The question for you is "what is the definition of driving in Ill.?" If it is not as stringent as CA, then the prior may be able to be successfully challenged and excluded in CA.
If the decision of the validity of the prior is left to the Court, the Court will simply look at the certified documents to determine what Ill. crime you pleaded to or were convicted of. Then, then the Court will look to see if the Ill. crime is substantially similar in its legal requirements to the crime of DUI in CA. If they are substantially the same and your constitutional rights were not violated in Ill., then the Court will likely sustain the validity of the Ill. prior. So, start with a good lawyer who will talk to the DA about whether it should really be a prior in CA.