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SPN 637 FMI 2

Engine Timing Sensor (Crankshaft) — 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: Crankshaft position/speed signal, the main reference for engine speed and timing. More on SPN 637.

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

HIGH — limit use, diagnose now

Can cause stalling or no-start.

Likely causes

  1. Faulty crank sensor
  2. Wrong air gap or debris
  3. Damaged flywheel tone ring
  4. Wiring fault

Checks & fixes (easiest first)

  1. Inspect sensor and connector
  2. Check resistance and AC output while cranking
  3. Scope the crank and cam signals
  4. Check for SPN 636 together

FAQ

What does SPN 637 FMI 2 mean?

Engine Timing Sensor (Crankshaft) — data erratic, intermittent, or incorrect. The signal jumps around, drops in and out, or disagrees with other information the controller has.

Is SPN 637 FMI 2 serious?

Can cause stalling or no-start.

What should I check first for SPN 637 FMI 2?

Inspect sensor and connector; Check resistance and AC output while cranking; Scope the crank and cam signals.