Heavy Commercial Diesel J1939 SPN / FMI Fault Code Diagnostic Directory
Complete SAE J1939 SPN and FMI heavy truck diagnostic trouble code database. Search diagnostic codes, symptoms, probable causes, ECM sensor pinout diagrams, and derate severity.
SPN 3251 FMI 2 / 0: Aftertreatment 1 DPF Differential Pressure Data Erratic or High
The Engine Control Module (ECM) detected an erratic or implausible reading from the Diesel Particulate Filter (DPF) differential pressure sensor delta-P lines.
- Cracked, pinched, or soot-clogged metal delta-P sensor sampling tubes
- Failed DPF differential pressure sensor (water intrusion in wiring harness pin 2/3)
ECM 120-Pin Connector Pin 44 (5V Ref), Pin 62 (Signal), Pin 81 (Sensor Return Ground)
View Full Step-by-Step Diagnostic Protocol →SPN 641 FMI 9: Variable Geometry Turbocharger (VGT) Actuator Abnormal Update Rate
Communication lost between the Engine ECM and the smart VGT electronic actuator over the dedicated J1939 powertrain sub-network.
- Open or shorted CAN-Bus twisted pair wires (Yellow/Green) at the 4-pin turbo actuator plug
- Blown 15A VGT unswitched battery fuse in the main PDM/battery box
VGT 4-Pin Deutsch: Pin A (Battery +12V), Pin B (Battery Ground), Pin C (CAN Hi), Pin D (CAN Lo)
View Full Step-by-Step Diagnostic Protocol →SPN 100 FMI 4: Engine Oil Pressure Sensor Circuit Voltage Below Normal
Oil pressure sensor signal circuit voltage dropped below 0.25 Volts DC, indicating an open circuit or dead short to ground.
- Signal wire chafed against cylinder block or oil pan rail
- Corroded 3-pin sensor connector terminal (water/oil ingress)
3-Pin Metripack: Pin A (5V DC Supply), Pin B (Signal 0.5-4.5V), Pin C (Low Reference Return)
View Full Step-by-Step Diagnostic Protocol →SPN 84 FMI 14: Wheel-Based Vehicle Speed Signal Special Instructions / Plausibility Error
ECM detected an erratic speed pulse count compared to transmission output shaft RPM and ABS wheel speed sensors.
- Excessive air gap or metal shavings on the transmission tailshaft magnetic speed sensor tip
- Damaged or broken twisted-pair VSS harness wiring along frame rail
2-Pin Weatherpack: Pin 1 (VSS + Signal), Pin 2 (VSS - Signal)
View Full Step-by-Step Diagnostic Protocol →SPN 107 FMI 3: Engine Air Filter Differential Pressure Circuit Voltage High
Air filter restriction sensor signal voltage exceeded 4.85 Volts DC, indicating an open circuit in the sensor return line.
- Disconnected harness connector at the air intake duct restriction switch
- Broken sensor ground wire in engine front harness
2-Pin Connector: Pin 1 (+5V / Signal), Pin 2 (Chassis Ground Return)
View Full Step-by-Step Diagnostic Protocol →SPN 3360 FMI 7: Diesel Exhaust Fluid (DEF) Controller Quality / Concentration Mechanical Not Responding
DEF quality sensor ultrasonic concentration measurement is outside the valid 32.5% ± 2.5% urea specification window or sensor head frozen/crystallized.
- Contaminated DEF fluid (water, diesel fuel, windshield washer solvent added to DEF tank)
- Failed DEF tank header assembly (integrated level, temperature, and quality ultrasonic sensor)
DEF Header 4-Pin: Pin 1 (Ignition +12V), Pin 2 (Ground), Pin 3 (J1939 CAN Hi), Pin 4 (J1939 CAN Lo)
View Full Step-by-Step Diagnostic Protocol →SPN 111 FMI 3: Engine Coolant Level Sensor Circuit High (Open Circuit)
ECM detected an open circuit in the coolant surge tank level probe circuit.
- Corroded or broken 2-pin connector at bottom of plastic coolant reservoir
- Oxidized stainless steel coolant sensor probe tips inside the surge tank
2-Pin Metripack: Pin A (Signal), Pin B (Sensor Ground Return)
View Full Step-by-Step Diagnostic Protocol →SPN 105 FMI 3: Intake Manifold Temperature Sensor Circuit Open / Shorted High
Intake manifold air temperature sensor signal voltage exceeded 4.90V, indicating open circuit or failed thermistor.
- Oil/soot soaked intake temperature thermistor element in intake elbow
- Broken wire in overhead engine sensor harness
2-Pin Weatherpack: Pin 1 (+5V DC Reference), Pin 2 (Signal to ECM Pin 28)
View Full Step-by-Step Diagnostic Protocol →High-Frequency Heavy Duty Diesel Diagnostic Trouble Codes
Click any diagnostic code below for full troubleshooting procedures, live sensor voltages, and ECM pinouts:
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