2014 Kenworth T800 check engine light

I have a 2014 Kenworth T800 with a Paccar MX13 engine. I was told the engine brake was not working so I took it out for a test drive and the engine brake was working fine but I noticed the check engine light come on when I would slow to a stop. The CEL light would go off when I would get back up to speed and then come back on when I parked it. I was not hooked up to any trailer and it did not go into derate. I do not have a scan tool but would like to purchase and affordable one that would work for Paccar, Detroit, and Cummins. Any info is appreciated, TIA.

 

Coolant level is good and all gauges reading correct. I've noticed that the CEL comes on when I let RPMs drop below 1000 coming to a stop sign. I can sit there and rev the engine to 1500 RPMs and the CEL goes off. If i go back down to 1000 RPMs it comes back on and rev up to 1500 RPMs it goes off. Curious if I have some sort of speed sensor going bad. TIA