T680 CEL lights on, keep running?

Depends on the severity of the issue causing them. Likely will not derate. Just something you should get checked as soon as possible because there is a possibility it could lead to larger problems if not dealt with.

 

What does it say on the diagnostic display?

 

If it is derated, it may tell you when you go through the diagnostic area of the in dash display. It will say something like "Derate". Then again, it may not and be only visible to a connected computer.

But driving it will tell you as well. Low power, pretty sluggish.

Yellow check engine lights aren't too much to get excited over. It's just a "Something needs checking" indicator. If you get ones that are red, then you have a real problem. They will literally shut you down.

 

If it's emissions related yellow can go to red reeaall quick. Or it could blink yellow for a month. It's a crap shoot.
In my experience a wrench light usually came with a derate and was a 'final warning'.
But unless you enjoy riding shotgun in a tow truck best check it out.

 

Just for future reference, The one with a wrench is a check engine light. The one without a wrench is the malfunction indicator lamp. The MIL indicates an emissions-related fault code (EPA requires certain failures of parts to illuminate an MIL). The CEL is everything else. It's rare on modern tractors that you'll have one light without the other.

Also, no one can tell you if the truck will derate based on which light is on. Some fault codes derate after 15 miles from first MIL illumination, some fault codes cause a derate 250 miles after the MIL illuminates.

 

Mine came on with a MIL only and it does that for a day or two and then goes away never derrates or acts up. I took it in to a shop and they said it was a oil temperature sensor went bad. The gauge for the front differential oil temp doesn't work so I think that might be it but why would it came up with a MIL light. Its a mistery to me. Any thoughts ?