Can anyone tell me if it's possible to check from ECU data when a particular fault code or warning first arose?

As long as it hasn't been cleared at any point, yes.

What other historical information can you obtain fro an ECU. For example what about distance travelled after fault arose?

What fault code are you interested in?

The freeze frame data will tell you the date, time and mileage when the fault first occurred.

Perhaps if you expanded a little on why you are asking it would help ......

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All to do with conversation in the pub. Guy was saying he bought car. Turned out it had a fault but dealer denied any knowledge. Check of ECU allegedly revealed fault had been evidenced some time back and warning light deactivated.

Not sure how much relevance can be put on claim but interesting to know what info might be stored on ECU.

Not sure how you would deactivate the warning light without clearing the faults (which also erases the freeze frame data) or by actually fixing the fault.

So if there is a historic error code with freeze frame data it is more likely that the fault is intermittant (light goes out but leaves the error code in memory) & the dealer didnt really check it before selling the car.