I have a 2010 Forester with nearly 100k miles, and it just started giving the "emissions" check engine code (P0420, Catalytic Converter Efficiency Below Threshold).
When that code is tripped in the system, it responds by flashing 'cruise' and solid light indicating the traction control system is disabled. These systems are disabled - turning at speed is noticeably worse, and cruise certainly doesn't work.
Why is that? Is there a reason to disable the traction control system and cruise control when the catalytic converter efficiency is below expected levels? If it's just a matter of having to do something to show the driver what kind of problem there is, why disable traction control (which is a valuable safety mechanism)?
One Subaru dealer offers a potential answer, though it is sufficiently vague to be more like marketing speak:
To work properly, the cruise control system and vehicle dynamics control system require a powertrain operating at peak efficiency. When the check engine light senses a problem, your Subaru deactivates those other systems to avoid erratic performance.
If that's the case, I'd love to know why this is so important - in particular, why vehicle dyanmics control ("traction control" above) is affected here.