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SPN 3216 FMI 4

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

What the SPN covers: NOx sensor reading in the exhaust before the SCR catalyst (engine-out NOx). More on SPN 3216.

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

MEDIUM — fix soon

Can lead to SCR efficiency codes and inducement derates if not repaired.

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. Failed NOx sensor (common)
  3. Sensor not reaching operating temperature
  4. Exhaust leak near the sensor
  5. Network or power fault to the sensor module

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. Check for exhaust leaks upstream of the sensor
  4. Check sensor power/ground and CAN wiring
  5. Compare inlet and outlet NOx readings
  6. Replace sensor per OEM test results

FAQ

What does SPN 3216 FMI 4 mean?

Aftertreatment 1 Intake NOx — voltage below normal or shorted low. The sensor circuit voltage is lower than it should ever be, usually near zero.

Is SPN 3216 FMI 4 serious?

Can lead to SCR efficiency codes and inducement derates if not repaired.

What should I check first for SPN 3216 FMI 4?

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; Check for exhaust leaks upstream of the sensor.