Home › Codes › SPN 3509 › FMI 2

SPN 3509 FMI 2

Sensor Supply Voltage 1 — 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: One of the regulated 5-volt reference supplies the engine computer feeds to a group of sensors. If it is pulled low or high, every sensor on that supply reads wrong at once. More on SPN 3509.

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

Several sensors dropping out together can cause derates, limp mode or a no-start. Find the short before replacing sensors.

Likely causes

  1. A shorted sensor dragging the supply down
  2. Reference wire chafed to ground or to battery voltage
  3. Water in a sensor connector
  4. ECM internal fault (less common)

Checks & fixes (easiest first)

  1. Note which other sensor codes set at the same time — they share the supply
  2. Unplug sensors on that supply one at a time and watch the voltage or code status
  3. Inspect harness routing near hot or moving parts
  4. Measure the reference at the ECM with sensors disconnected

FAQ

What does SPN 3509 FMI 2 mean?

Sensor Supply Voltage 1 — data erratic, intermittent, or incorrect. The signal jumps around, drops in and out, or disagrees with other information the controller has.

Is SPN 3509 FMI 2 serious?

Several sensors dropping out together can cause derates, limp mode or a no-start. Find the short before replacing sensors.

What should I check first for SPN 3509 FMI 2?

Note which other sensor codes set at the same time — they share the supply; Unplug sensors on that supply one at a time and watch the voltage or code status; Inspect harness routing near hot or moving parts.