SPN 639 FMI 2 — J1939 Data Link — Communication Error
What it means: The J1939 CAN datalink — the network that lets the engine, transmission, ABS, and dash talk — has a communication error.
How serious / can you keep driving: Caution. Limp to the next safe stop.
Check first: Note WHICH modules dropped — a pattern points to where on the backbone the break is.
Don't risk it — look up listed providers near you and give them this SPN/FMI when you call.
Listed providers nearest Sparks, NV
Closest first. Tell them SPN 639 FMI 2 and what the truck is doing. RoadService.app lists these providers and does not employ them or carry out the repair.
- 0.0 mi away — based in Sparks, NV · (775) 355-6884
- 0.0 mi away — based in Sparks, NV · (775) 356-7200
- 0.0 mi away — based in Sparks, NV · (775) 359-0550
What SPN 639 FMI 2 means
The J1939 CAN datalink — the network that lets the engine, transmission, ABS, and dash talk — has a communication error. Modules drop offline, gauges can go blank, and seemingly unrelated systems act up. It's a wiring/network issue more often than a dead module.
How serious is it?
Caution. Limp to the next safe stop.
Most likely causes
- Damaged J1939 backbone wiring, a chafed twisted pair, or a bad connector
- Missing or failed 120-ohm terminating resistor (datalink not properly terminated)
- A single module shorting or flooding the bus
- Corrosion at a diagnostic/inline connector on the datalink
First moves (roadside)
- Note WHICH modules dropped — a pattern points to where on the backbone the break is.
- Check both 120-ohm terminating resistors (you should read ~60 ohms across CAN-H/CAN-L, key off).
- Inspect datalink connectors (including the diagnostic port) for corrosion or backed-out pins.
- Look for recent harness work, rodent damage, or chafe points near the break area.
Common misdiagnoses
- Replacing a module that merely went silent because the bus dropped.
- Ignoring a missing/failed terminating resistor, the classic intermittent-comm cause.
- Blaming the dash when the real fault is upstream wiring.
FAQ
The J1939 CAN datalink — the network that lets the engine, transmission, ABS, and dash talk — has a communication error. Modules drop offline, gauges can go blank, and seemingly unrelated systems act up. It's a wiring/network issue more often than a dead module.
Caution — Limp to the next safe stop.
The most common cause is: Damaged J1939 backbone wiring, a chafed twisted pair, or a bad connector. Other possibilities include Missing or failed 120-ohm terminating resistor (datalink not properly terminated); A single module shorting or flooding the bus; Corrosion at a diagnostic/inline connector on the datalink.
Limp to the next safe stop. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Related codes
Codes that commonly appear alongside SPN 639 FMI 2, or share the same subsystem.
- ECU Power Supply — Intermittent / Erratic (SPN 627 FMI 2)
- Battery / Charging Voltage — Low (SPN 168 FMI 18)
- Engine Controller (ECM) — Internal Fault (SPN 629 FMI 12)
- Battery voltage low during crank (SPN 168 FMI 17)
- J1939 proprietary datalink lost (SPN 625 FMI 9)
- High Speed CAN Bus Fault (U0001)
- Lost Communication with ECM/PCM (U0100)
- Lost Communication with Fuel Injector Control Module (U0105)
Same symptom: Lost communication
Other datalink faults. A U-code is a network problem and is diagnosed at the datalink, not at the module that went quiet.
- Lost Communication with TCM (U0101) · Electrical, Charging & Network
- Lost Communication with Gear Shift Module (U0103) · Electrical, Charging & Network
- Lost comms with ABS (U0121) · Electrical, Charging & Network
- Lost Communication with Body Control Module (U0140) · Electrical, Charging & Network
- Invalid Data from ECM (U0401) · Electrical, Charging & Network
- Invalid Data from TCM (U0402) · Electrical, Charging & Network
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.