SPN 639 FMI [9,13] is a diagnostic trouble code for J1939 Network #1, Primary Vehicle Network
FMI 2 – Data Erratic
FMI 8 – Abnormal Frequency
FMI 9 – Abnormal Update Rate
FMI 13 – Out Of Calibration
FMI 19 – ECU internal fail
ECM connections, fuses, wiring harness
I had been driving trucks for a good part of a decade, but this code was not one I had memorized. The SPN 639 pertained to the Controller Area Network (CAN) Bus, a vital network that allows various components of the vehicle to communicate with each other. I knew that much, but the specifics of the error were beyond my immediate knowledge.
I pulled off the highway and into a rest area, parking my rig between two other long-haulers. The engine, a 13-liter behemoth that typically boasted 475 horsepower, now idled with what I imagined was an electronic whimper. I fetched my laptop from the side compartment of the driver’s door, which was about the size of a small briefcase and had seen better days, and started scouring the internet for more information.
The details I found were technical: SPN 639 indicated a fault with the CAN Bus’s communication, but it could be triggered by a myriad of issues—from a simple loose connection or blown fuse to a more serious malfunctioning electronic control unit (ECU). I popped open the fuse box, a small plastic cover no larger than a standard letter envelope, and checked each fuse. They all seemed fine.
With the fuses in good shape, I realized the problem was likely deeper than I could diagnose on my own. The CAN Bus system was like the nervous system of the truck, a complex web of wiring that snaked its way through the vehicle’s frame. I wasn’t about to start taking apart the dash and digging through the guts of my dashboard on the side of the road.