My Check engine warning light just came on on my 2000 Grand Vitara. Last August, it also came on (then cycled on/off every 300 miles or so until the cold the weather hit in November). It has never come on since, but now, August again it is on again. It came on while driving at highway speed, no traffic, and the temp gauge was normal, but I can't help but thinking it's (outside) temperature related. Would a combination of temp/humidity (and maybe time exposed to these conditions) trigger warning light?

 

No

It's coming on and off to let you know the computer has detected something that needs attention
It's telling you it's not something vital right now important (constant light), but it does have some trouble codes which you should have checked out

It will go out by itself if the problem is fixed, "goes away", or doesn't appear for a certain amount of time (on/off cycles actually)

So it is possible that the problem the computer detected gets worse in hot weather, which is different than the weather causing the light to fire up

The computer still has a problem code or two for you
It's not something that will cause failure right away, but it could be a problem that will cost more money to fix later on down the road
Many DIY Auto Parts places will read the codes for a small fee or no charge

Just don't buy a part w/o posting up the exact code(s) here for interpretation
(An "O2 sensor reads lean" code does not mean the o2 sensor is bad)