When diagnosing SPN 4355/FMI 5, it typically indicates a Line Heater 2 Open Circuit issue in the Diesel Exhaust Fluid (DEF) system. This guide outlines the diagnostic steps to identify and resolve the problem efficiently.
1. Check for Multiple Codes
- If SPN 4355/FMI 5 and SPN 4357/FMI 5 are both present, inspect and repair the open circuit between pin 15 of ACM2.1 (120-pin connector) and pin 2 of line heaters 2 and 4.
- If only SPN 4355/FMI 5 is present, proceed to step 2.
2. Disconnect Line Heater 2
Ensure the line heater 2 is completely disconnected before proceeding with further tests.
3. Turn Ignition ON
Turn the ignition key to ON while keeping the engine OFF.
4. Measure Voltage Between Pin 2 of the Line Heater 2 Connector and Ground
- If the voltage is greater than 11.75V, proceed to step 5.
- If the voltage is less than 11.75V, repair the wiring between pin 2 of line heater 2 connector and pin 15 of ACM2.1.
5. Measure Voltage Between Pin 1 of the Line Heater 2 Connector and Ground
- If the voltage is between 2.25V and 2.50V, go to step 6.
- If the voltage is outside this range, repair the wiring between pin 1 of the line heater 2 connector and pin 24 of ACM2.1.
6. Measure Resistance of the Heating Element in Line Heater 2
If the resistance is out of the specified range, replace the line heater. Refer to the resistance values in the table below.
Diesel Exhaust Fluid Line Heater Resistance Chart
| Length (mm) | Length (inches) | Nominal Resistance (Ω) | Min Resistance (Ω) | Max Resistance (Ω) |
|---|---|---|---|---|
| 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
Following this step-by-step diagnostic guide will help technicians effectively troubleshoot SPN 4355/FMI 5 errors related to DEF Line Heater 2. Checking voltage, wiring, and resistance ensures proper function and avoids unnecessary replacements. Regular maintenance and timely repairs prevent DEF system failures, improving vehicle performance and emissions compliance.
