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SPN 639 FMI 2

J1939 Network #1 (Primary Data Link) — Data erratic, intermittent, or incorrect.
The signal jumps around, drops in and out, or disagrees with other information the controller has.

What the SPN covers: The main J1939 CAN data link that modules use to talk to each other. More on SPN 639.

What FMI 2 usually points to: Loose or corroded connectors, chafed wiring, a failing sensor, or two redundant sensors that do not agree. More on FMI 2.

Severity

MEDIUM — fix soon

Can cause many modules to lose data, dash gauges to drop, and derates.

Likely causes

  1. Missing or failed terminating resistor
  2. Shorted or open CAN wires
  3. A failed module dragging the bus down
  4. Damaged diagnostic connector or aftermarket device

Checks & fixes (easiest first)

  1. Measure resistance across CAN high/low at the diagnostic port (key off, about 60 Ω expected)
  2. Unplug modules one at a time to find a bad one
  3. Inspect for chafed twisted pair
  4. Remove aftermarket devices on the bus

FAQ

What does SPN 639 FMI 2 mean?

J1939 Network #1 (Primary Data Link) — data erratic, intermittent, or incorrect. The signal jumps around, drops in and out, or disagrees with other information the controller has.

Is SPN 639 FMI 2 serious?

Can cause many modules to lose data, dash gauges to drop, and derates.

What should I check first for SPN 639 FMI 2?

Measure resistance across CAN high/low at the diagnostic port (key off, about 60 Ω expected); Unplug modules one at a time to find a bad one; Inspect for chafed twisted pair.