Engine code scanner - check engine light not on
 

This is kind of a random question me and my friend were discussing. We were wondering if something were awry with the engine but the check engine light wasn't on if a scanner would still let you know. He is getting a feeling something is wrong with his truck but his check engine light hasn't illuminated.

Well the answer is maybe!!!!
"Pending" codes can be stored & the CEL not be lit.

Other things can be wrong, but if the computer isn't monitoring them, a CEL won't be lit, nor a pending code stored, Unless the problem adversley affects something that IS being monitored by the computer.

SO, if he has a "scantool", that'll monitor live PID feeds to the computer, he might be able to get an idea of whats going on by reviewing the PID's the scantool can monitor, to see if any parameters look abnormal.

+1....I have solved a few issue by "catching" a pending code...........also make sure the CEL comes on when you first turn the key to ON/RUN

Also if something with your Truck is "wrong" the first thing you should do is bring all the maintenance up to Par.....a lot of times that solves the issue...

+1 on both of these...I've seen this before as well, and doing any past due maintenance is a great idea...