Home › Codes › SPN 636 › FMI 8

SPN 636 FMI 8

Engine Position Sensor (Camshaft) — Abnormal frequency, pulse width, or period.
A frequency- or PWM-type signal is outside the expected range or shape.

What the SPN covers: Camshaft position signal used for injection timing and cylinder identification. More on SPN 636.

What FMI 8 usually points to: Damaged speed sensor or tone ring, wrong air gap, electrical noise, or a failing PWM sensor. More on FMI 8.

Severity

HIGH — limit use, diagnose now

May cause hard starting, rough running or no-start.

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. Faulty cam sensor
  3. Wrong air gap or debris on the sensor
  4. Damaged tone ring
  5. Wiring fault

Checks & fixes (easiest first)

  1. Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins
  2. Check the reference voltage, ground and signal at the component per OEM pin-out
  3. Inspect the sensor tip for metal debris
  4. Check resistance and wiring
  5. Compare cam and crank signals with a scope
  6. Check for SPN 637 together

FAQ

What does SPN 636 FMI 8 mean?

Engine Position Sensor (Camshaft) — abnormal frequency, pulse width, or period. A frequency- or PWM-type signal is outside the expected range or shape.

Is SPN 636 FMI 8 serious?

May cause hard starting, rough running or no-start.

What should I check first for SPN 636 FMI 8?

Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins; Check the reference voltage, ground and signal at the component per OEM pin-out; Inspect the sensor tip for metal debris.