These things happened after running over a small  tree branch with my 09 Impreza. Pressing control buttons had no effect.  After a day's worth of stop/restarts, the problem disappeared. I cleared a P0420, and none of the lights came back. I thought maybe a wiring issue--your thoughts?

I think there is a 8 year/80,000 mile warranty on emissions related items. Get it on the record if you fit within those guidelines and don't bother to mention the branch. 

Not sure that a p0420 would result from a reversible wiring fault.

exhaust leaks can cause that code - maybe have the car examined for damage underneath - you could probably put in an insurance claim but with deductible.....?

Possible you damaged one of the catalytic converters, or one of the oxygen sensors.

 

On newer Subarus when the check engine light comes on, you also get a flashing cruise control light because the cruise control is disabled when the CEL is on. Traction control may also be disabled. Sometimes the traction control light may come on due to a wheel slipping or losing contact with the ground if something is run over.

Checked underside and found no obvious exhaust or sensor damage, and the P0420 hasn't come back. Just curious how the VDC and cruise issues rectified themselves.

Systems often disable with a check engine light, so the P0420 caused the VDC and cruise to disable. 

Once that code disappeared those systems resumed nominal ops. 

 

why the P0420 code was set might be the main question.  That's weird.