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Is there a way, for a given car model, without physical access to a car of this exact model, to find what data are logged, what is the frequency at which they are logged, what the size of the log is, and so on. At least the default values, if this is something that can be altered. Let's say, Audi A3 for example.

Also, is there a place where I could find actual raw OBD-II data ?

  • What do you mean by "logged?" There is bus activity, that is constant but not stored anywhere, and there are diagnostic codes of which some are stored under certain conditions. What are you trying to do with this information? – Sam Basso Feb 1 '18 at 18:59
  • I was wondering if there is some storage where data are recorded. That's all. I assume some data are recorded. – alecail Feb 1 '18 at 19:05
  • Cars are not fitted with a "black-box" like aircraft are... they can store fault codes and some readers will give instantaneous data readings. – Solar Mike Feb 1 '18 at 21:27
  • I think another way of stating your question in the more car centric language is what PIDs are supported on my vehicle. – jxramos Feb 1 '18 at 22:52
  • There are no data logged, aside from the mentioned "Freeze Frame" data, which only occurs relative to an instant snapshot of a fault code event. Live raw PID parameters are available at the OBD-II port based on OEM preference, but there is no law or rule as to what PIDs must be supported. Although in modern implementations the data are usually fairly feature-rich. – SteveRacer Feb 2 '18 at 3:53
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The only user accessible "logging" that happens is called "Freeze Frame Data" that is normally only saved when certain (usually emissions related) trouble codes are set.

The OBDII Freeze Frame is considered one frame of information taken by an on-board data recorder. When a fault occurs, the PCM stores the input data from various sensors so that technicians can determine under what vehicle operating conditions the failure occurred.

From: http://www.checkenginetroublecode.com/obdii-freeze-frame-explained/

So, what you get is a reading from several sensors that took place at a single time. It's not a running log, and it's not all sensors. It's very, very limited, and the data that is recorded changes based on the trouble code that was set.

That said, if you want to log data, you can log anything you want on your A3 with a cable, a laptop, and software called VCDS (aka. VAG-COM).

  • There are also OBD-2 plug in sensors you can use with a smartphone app (wifi or bluetooth) to capture logs of the data and see other data in almost real time. – Chris Paveglio Feb 1 '18 at 21:03
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    @ChrisPaveglio, that is correct, but for an Audi or VW, VCDS is a factory-level scan tool that lets you record everything - not just what is exposed by the generic OBDII interface. OP asked for "raw" data, and VCDS will expose it. Of course, he might be happy with either method, so good point. – JPhi1618 Feb 1 '18 at 21:12
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    "...the data that is recorded changes based on the trouble code that was set." I find this to be untrue, in my experience (Domestic and Asian) the manufacture decides which PIDs to capture and they always capture the same PIDs regardless of the DTC set. Newer cars (CAN on-wards) do a better job exposing more PIDs. – Ben Feb 1 '18 at 22:15
  • @Ben, thanks for that - I had heard that some PIDs will record different data, but in practice I've seen the same data as you say. – JPhi1618 Feb 2 '18 at 14:22
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Most cars made after 2014 have an EDR or event data recorder. There is not a lot of information available as to how long an event cycle is, or exactly what data is stored. As cars have gotten more sophisticated with things like ABS and Automatic Stability Control, the control systems require more inputs and sensors. Things like steering wheel angle, body roll and individual wheel speed, throttle position, airbag activation and number of occupants are all monitored. Who has access to the data varies by location at least in the U.S. Most states require the owners permission or a court order, but not in all states. I believe currently the information is generally only available with the assistance of the manufacturer.

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    This is usually only "crash" or impact/airbag deployment data. While the scope of the OP's question is unclear, your answer is indeed mostly correct (since 2010 the CDR data is mandated to be available and OEMs must offer the software/tool if needed). However, I suspect the question was directed about continuous ECM PIDs. – SteveRacer Feb 2 '18 at 3:36
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Just to comment on a concrete value for sampling rate I have a 2005 Volvo S80 and have found my sampling rate for the live recording of 5 PIDS to be about 2Hz. You can see more at my question asked here: Does limiting the number of PIDs on an OBDII Live Scan increase the sampling rate?

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    I've seen the same kind of [lack of] performance with Autel handhelds. Certainly there is a limit to the bus speed, and more PIDs monitored means more data. However, my Snap-On Solus performs much better, with an apparent sample rate of 4 or 5 times that. Interestingly, graphs seem to be much more lively than raw numbers. Ultimately, I suspect the limitations are combination of the scan tool power, the ECM priority of the diagnostic task, and the bus protocol and speed. It's no surprise to me that my $10000 Solus is faster than my $150 Autel. It should be! But not 66 times faster... – SteveRacer Feb 2 '18 at 3:46

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