According to a report that I saw the only thing that being a subscriber does is allow the user to access the features of the system. Whether the subscription is active or not has no bearing on OnStar's ability to communicate with the car, it is always accessible to them. If the module is still connected and not defective they can do this regardless of whether there is a subscribtion or not.
I have no reason to doubt this based on the experience with my wife's car, which is equipped with OnStar. We have never subscribed to or paid for the service since the day that she bought the car. It is five years old now. The dealer does not do the oil changes, I do. Despite that we still get messages from the dealer that it was purchased from advising us that the life remaining on the engine oil is down to 15%, 10%, 3%, whatever it may be, and suggesting that we call them to schedule an appointment for an oil change. If I check the on-board oil life system it matches the percentage that they quoted. The only way that they could know this is if the system is sending this data to OnStar, who is sending it to GM, who is forwarding it on to the dealer. The fact that we do not pay for or have an active subscription obviously does not prevent communication between our car and OnStar.
OnStar is owned by GM. Ever wonder why it's not offered on other vehicle brands, unless they have a GM agreement? *The old OnStar FMV units excluded*
Of interesting note in the above wiki...
"
In September 2011, OnStar has changed its terms and conditions to allow sale of vehicle location and speeds to interested third parties including law enforcement agencies, which has been criticized in the
comp.risks forum.
[25]
Use as surveillance device
OnStar is theoretically susceptible to remote activation by malicious third parties or government agencies. This activation would enable third parties to track the location of the car, along with the ability to listen to the contents of any conversations carried on by the occupants within the car without their consent. However, the
FBI has denied the ability to use this as it disables OnStar's safety features, as determined by the
Ninth Circuit Court of Appeals.
[26] In its document of privacy practices, OnStar states that it is not possible for them to listen to or monitor conversations in a car without the knowledge of the occupant. The hardware is designed so that when an advisor calls into a car, a light flashes, a ring tone is heard, and the radio will mute.
[27]
In 2011, OnStar said that it would start retaining all the information collected by the GPS and internal system so that it could be sold to third parties (possibly insurance companies).
[28] Although this data is supposed to be "anonymized", exactly what OnStar means by anonymizing GPS data remains unclear, and it may be impossible to make the data actually anonymous.
[29] A few weeks later, after outcry from subscribers and privacy advocate groups,
[30] OnStar reversed the decision
[31] to continue collecting information from unsubscribed units.
A 2019 experiment hacking into the data collected by a 2017 Chevrolet Volt revealed that the car stored logs of calls made from the user's cell phone, as well as location and driving data, which could then be sold to third parties.
[32]
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