CEL that doesn't save?

[cooolkid]

I'm getting a check engline light sometimes while driving, but when I check for codes it isn't saved? What are some causes of realtime cel?

 

[benchwarmer]

That happened to me, turned out to be the knock sensor wiring. I did the knock sensor rewire and haven't had the problem since.

 

[ben1984j]

Code 51 is a real-time error code, and I'm pretty sure it's the only one...