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SPN 3242 FMI 4

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

What the SPN covers: Exhaust temperature entering the diesel particulate filter. More on SPN 3242.

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

Regenerations may be blocked until fixed, leading to soot buildup.

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. Failed exhaust temperature sensor
  3. Wiring damage from heat
  4. Diesel oxidation catalyst not lighting off

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. Compare all exhaust temperature sensors at idle after a cold soak
  4. Inspect wiring for heat damage
  5. Check DOC performance during a regen

FAQ

What does SPN 3242 FMI 4 mean?

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

Is SPN 3242 FMI 4 serious?

Regenerations may be blocked until fixed, leading to soot buildup.

What should I check first for SPN 3242 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; Compare all exhaust temperature sensors at idle after a cold soak.