Home › Codes › SPN 3509 › FMI 4

SPN 3509 FMI 4

Sensor Supply Voltage 1 — Voltage below normal or shorted low.
The sensor circuit voltage is lower than it should ever be, usually near zero.

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 4 usually points to: Signal wire shorted to ground, a lost 5 V reference, or an open signal wire on a pulled-down circuit. More on FMI 4.

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. Wiring or connector fault in this circuit (most common for electrical FMIs)
2. A shorted sensor dragging the supply down
3. Reference wire chafed to ground or to battery voltage
4. Water in a sensor connector
5. ECM internal fault (less common)

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

FAQ