[paulmlewis]

I think the majority of modifications are simply adjusting the calibration or lookup tables for things like fuel maps, enabling / disabling things like key left in ignition buzzers and the like, mostly data items rather than code modifications. There may be more adventurous modifications like adding launch control to a performance cars, but that may just be enabling existing code rather than adding new code.

My cars are old (1998) Nissan Skylines, who's ECUs are pretty basic, asides from killing the engine there's not much you can do to cause more issue than a mechanical modification like adding a larger turbo, or maintenance neglect. The ABS and traction control are handled by physically separate ECUs, though I imagine things are more integrated in the main drive train ECU in modern cars.

While I haven't modified the existing or written my own firmware to load on the ECU that mostly due to lack of time, one of them came from Japan with a Piggy Back ECU installed which intercepts the inputs / outputs to override the mapping of the OEM ECU to tune for other mechanical modifications. An alternative is to buy an aftermarket ECU or build a custom one, those tend to have less integrated safety features (stability control, etc.) than the OEM ones and integrate less well with the car's other systems, I'd expect them to cause more issues overall than relatively simple modifications of the OEM firmware.

I'm sure there are extremes where people may cause problems, but this kind of thing has been happening since cars have had computers so I doubt there's any great calamity around the corner.

[alkonaut]

I think there are still issues with even otherwise "safe" modifications. The road tax for a specific car model is (or should) be set based on emissions. Just like you would be fined if you were pulled over and had left your catalytic converter at home, it could be considered illegal to modify the fuel maps of a car to a higher power one, for example (At least if it hadn't been inspected and its tax adjusted after the modification).

[paulmlewis]

That's a fair point, my cars are old enough to come under the older UK road tax rules so are not taxed based on emissions, but their emissions are better than the requirements for their age.

Still it would be nice if there were inexpensive ways to check emissions for your self while tinkering.

However, I think fuel maps vs. catalyst removal are largely similar, it'll either be picked up on the next inspection or VOSA can do spot checks if they think something is up. I don't think DRM / technical measures for locking the ECU are appropriate much like I wouldn't be happy if the catalyst and exhaust system were installed such that only the manufacturer could replace them.

Perhaps one would only use a performance map when using the car on a race track, it'd be a shame to replace all the electronics just for that.

I'm also not a fan of things like geo-fenced speed limiters, and replacement components that have to be coded to the car by the dealer.