My check engine light has been coming on after cruising between 55mph and 60mph for about 10 minutes. This had actually been going on for the last 50,000 miles. But, once I turn the van off and back on the CEL stays off. I finally pulled the codes today and I get a #51.

Things to note right now.
- AC belt is not on.
- Reverse lights don't work due to a bad neutral start switch.

Based on the above notes code 51 makes sense right?

But this doesn't help diagnose the CEL after 55mph to 60mph cruising. Thoughts?

The van runs perfect so I'm not worried. It's just bugging me.