Understanding SPN 4356/FMI 5 Fault Code
SPN 4356/FMI 5 is a diagnostic trouble code (DTC) indicating a Line Heater 3 Open Circuit in the Diesel Exhaust Fluid (DEF) system. This issue can affect DEF dosing and engine performance, making it crucial to diagnose and fix promptly.
Step-by-Step Troubleshooting Process
1. Check for Multiple Fault Codes
- If SPN 4356/FMI 5 and SPN 4354/FMI 5 appear together, repair the open circuit between pin 9 of the ACM2.1 120-pin connector and pin 2 of line heaters 3 and 1.
- If only SPN 4356/FMI 5 is present, proceed to the next step.
2. Disconnect Line Heater 3
Unplug the connector to isolate the heater and prevent interference with diagnostics.
3. Turn Ignition ON (Engine OFF)
Powering the system allows for voltage testing.
4. Measure Voltage at Pin 2 of the Line Heater 3 Connector
- If the voltage is greater than 11.75V, proceed to Step 5.
- If the voltage is less than 11.75V, repair the wire between pin 2 of the line heater 3 connector and pin 9 of the ACM2.1 module.
5. Measure Voltage at Pin 1 of the Line Heater 3 Connector
- If the voltage is between 2.25V and 2.50V, move to Step 6.
- If the voltage is outside this range, repair the wire between pin 1 of the harness side of the heater 3 connector and pin 18 of the ACM2.1 module.
6. Measure the Resistance of Line Heater 3
If resistance values fall outside the acceptable range, replace the line heating element. Refer to the resistance chart below for proper specifications.
Diesel Exhaust Fluid Line Heater Resistance Chart
| Length (mm) | Length (inches) | Nominal Resistance (ohms) | Min Resistance (ohms) | Max Resistance (ohms) |
|---|---|---|---|---|
| 500 | 19.6 | 16 | 12.8 | 19.2 |
| 1000 | 39.3 | 9.8 | 7.84 | 11.76 |
| 1200 | 47.2 | 8.6 | 6.88 | 10.32 |
| 1400 | 55.1 | 7.8 | 6.24 | 9.36 |
| 1600 | 62.9 | 7 | 5.6 | 8.4 |
| 1800 | 70.8 | 6.1 | 4.88 | 7.32 |
| 2000 | 78.7 | 5.6 | 4.48 | 6.72 |
| 2200 | 86.6 | 5.2 | 4.16 | 6.24 |
| 2400 | 94.4 | 4.9 | 3.92 | 5.88 |
| 2600 | 102.3 | 4.6 | 3.68 | 5.52 |
Conclusion
By following these diagnostic steps, mechanics and fleet operators can accurately identify and resolve SPN 4356/FMI 5 faults. Regular maintenance and inspections of the DEF line heaters help prevent failures and improve engine efficiency. If resistance values do not align with the table above, replacing the heating element is recommended to maintain optimal DEF system operation.
