Q. The “check engine soon” light comes on when I’m towing my KZ fifth-wheel trailer. I have a 2001 GMC 2500HD 4WD with a Duramax diesel. The truck has been checked twice by a GMC dealer and once by a private shop. A boost code keeps coming up, but when checked everything appears OK. The only time the engine light comes on is when the truck is under load and going uphill. Once the load is lightened, the light shuts off, only to happen again on the next hill. Any ideas?
— A.L., Corpus Christi, Texas
A. There are several boost-related codes. Each trouble code has certain criteria to turn the malfunction indicator lamp on. On codes that are emission-related, there should be “freeze frame” data, which is data captured when the code gets turned on. This allows the technician to duplicate the conditions that set the code. If that doesn’t work, then what you need to do is ask the dealership to install a data recorder or “flight recorder.” Then, you can drive it with the fifth-wheel, get the light to come on, and then capture the data.
Ken Freund’s more than three decades of auto-repair experience and 20-plus years of RVing helped him author numerous books and articles on vehicle repair. In addition to RV Clinic and Performance, he writes the Powertrain column in MotorHome magazine. Ken has been a California Automotive VO-Tech and Smog-Test Program Instructor and an ASE-certified Master Automobile Technician.