A couple months back, I was have a "check engine light" issue as you describe on my 2004, cat 3126. It was frustrating not knowing when it would go into the "limp mode" Took it to a Freighterliner shop in Gilbert, AZ. They ran the codes and did a road test and could not come to a conclusion. Weeks latter and just outside of Denver, CO, I got fed up and took it to another Freightliner shop. After reading the codes they stated that the "Injection Actuation Pressure Control Sensor" is bad and needs to be replaced. 2 weeks and 1500 miles latter the Cat runs great.
