Fault Code SPN 4355 indicates an open circuit condition in line heater 2 of the diesel exhaust fluid (DEF) system on vehicles equipped with selective catalytic reduction (SCR). When this fault appears, the DEF may not be heated properly which can impact emissions and lead to other issues. In most cases, you can continue driving the vehicle but the fault should be diagnosed and repaired as soon as possible to avoid potential damage or further problems.
In our workshop, we've encountered SPN 4355 faults across various makes and models, but it seems to be more prevalent in European vehicles. Just last week, a Mercedes-Benz Actros came in with the check engine light on and derated performance. Scanning the fault codes revealed SPN 4355 FMI 5, indicating an open circuit in DEF line heater 2. We've seen similar issues before on MAN, Volvo, and DAF trucks as well. The root cause is often corroded wiring or a failed heating element. In most cases, we can clear the code and get the truck back on the road the same day by repairing the wiring or replacing the heater. While it's not a crippling fault, it's important to address promptly to avoid further issues with the DEF system and potential downtime.
Understanding SPN 4355
SPN 4355 is a generic J1939 fault code that applies to any vehicle using the SAE J1939 protocol, not a specific make or model. The code indicates:
- SPN 4355 – Suspect Parameter Number referring to line heater 2
- FMI 5 – Failure Mode Indicator meaning open circuit
The DEF line heaters are electrical resistance elements that warm the DEF to prevent freezing and maintain optimal fluid characteristics for injection into the exhaust stream. They are controlled by the Aftertreatment Control Module (ACM). Line heater 2 is located between the DEF tank and pump.
Common Causes
Some common causes that can trigger SPN 4355 include:
Buy tested tuning file for Adblue / EGR / DPF / Adblue off now!
- Damaged wiring or connectors to line heater 2
- Defective line heater element
- Malfunctioning ACM
- Improper DEF fluid level (too low or overfilled)
Diagnosis
Follow these steps to diagnose SPN 4355:
-
Check for other fault codes. If SPN 4355 and 4357 are both present, suspect an open between the ACM and heaters 2 and 4.
-
Disconnect line heater 2 and turn ignition on. Measure voltage between pin 2 on harness side and ground.
- If >11.75V, go to step 3.
- If <11.75V, repair open between pin 2 and ACM pin 15.
-
Still at line heater 2 harness, measure voltage between pin 1 and ground.
- If 2.25-2.50V, go to step 4.
- If not 2.25-2.50V, repair open between pin 1 and ACM pin 24.
-
Measure resistance of line heater 2 element. Replace if not within specs in table below.
| Length (mm) | Length (in) | Nominal Ω | Min Ω | Max 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 | 7.0 | 5.6 | 8.4 |
| 1800 | 70.8 | 6.1 | 4.88 | 7.32 |
| 2000 | 78.7 | 5.6 | 4.48 | 6.72 |
Repair Solutions
Repairs for SPN 4355 typically involve:
- Fixing wiring issues like repairing opens or shorts in the harness to the line heater
- Replacing the line heater element if resistance is out of spec
- Replacing the ACM if it is defective
- Adjusting DEF level if overfilled
Always refer to the OEM service information for specific repair instructions, wiring diagrams, and part numbers.
Repair Costs
Repair costs will vary depending on the root cause but may include:
- Line heater element – $100-$200 (€90-€180)
- Wiring repairs – $50-$150 (€45-€135)
- ACM replacement – $500-$1000 (€450-€900)
- 1-3 hours labor at local shop rates
Prevention
To help prevent SPN 4355 from occurring:
- Maintain proper DEF levels, do not overfill
- Inspect wiring and connectors regularly for damage
- Use high-quality DEF fluid from reputable suppliers
- Address any other DEF system faults promptly
Related Problems
Other faults that may appear along with SPN 4355 include:
- SPN 4357 – Line heater 4 open
- SPN 4354/4356 – Line heaters 1/3 open
- SPN 3031/4094 – DEF tank level sensor issues
- SPN 4334 – Blocked DEF line
- SPN 3361 – DEF pump malfunction
Frequently Asked Questions About SPN 4355
What is SPN 4355?
SPN 4355 is a generic diagnostic trouble code that indicates an open circuit condition in line heater 2 of the diesel exhaust fluid (DEF) system. The DEF line heaters are electrical resistance elements that warm the DEF to prevent freezing and maintain optimal fluid characteristics for injection into the exhaust stream.
What does FMI 5 mean in relation to SPN 4355?
FMI 5 indicates an open circuit condition. So SPN 4355 FMI 5 specifically points to an open circuit in line heater 2 of the DEF system.
What are the symptoms of SPN 4355?
Common symptoms include:
- The check engine light illuminating
- DEF quality warnings or inducement of limp mode
- Possible increase in NOx emissions
- In cold weather, DEF freezing in the lines
Can you continue driving with SPN 4355?
In most cases, you can continue driving the vehicle for a short time but the fault should be diagnosed and repaired promptly to avoid potential damage or further issues. Continuing to operate the vehicle with the DEF system not functioning properly can lead to more serious problems.
What component is line heater 2 in the DEF system?
Line heater 2 is located between the DEF tank and pump. Its purpose is to warm the DEF in that portion of the line to keep it from freezing and to maintain the proper fluid temperature and viscosity for injection.
What are common causes of SPN 4355?
Common causes include:
- Damaged wiring or connectors to line heater 2
- Defective line heater element
- Malfunctioning DEF system control module
- Improper DEF fluid level (too low or overfilled)
How do you diagnose SPN 4355?
Diagnosis typically involves:
- Checking for other codes. SPN 4355 and 4357 together indicate an issue between the controller and heaters 2 and 4.
- Disconnecting line heater 2 and checking for voltage and resistance at the pins.
- Comparing measured resistance of the heater element to specifications.
- Inspecting wiring and connectors for damage.
What repairs are needed for SPN 4355?
Repairs depend on the diagnosed root cause but may involve:
- Replacing a damaged line heater element
- Repairing wiring issues like opens or shorts in the harness
- Replacing a faulty DEF controller
- Adjusting the DEF level if overfilled
What is the typical cost to repair SPN 4355?
Repair costs vary but common estimates include:
- Line heater element: $100-$200
- Wiring repairs: $50-$150
- DEF controller replacement: $500-$1000
- 1-3 hours labor: $100-$600
Total costs are often in the range of $200 on the low end up to $1500 for more involved repairs.
Is SPN 4355 specific to certain makes and models?
No, SPN 4355 is a generic code that can apply to any diesel vehicle using Selective Catalytic Reduction (SCR) with DEF for emissions control. However, the exact repair procedures and part numbers will vary by make and model.
Can SPN 4355 cause drivability issues?
Usually SPN 4355 alone will not cause noticeable driving issues, but it can lead to problems with the DEF system that may eventually result in loss of power or forced low-speed operation if the issue is not resolved.
How can you prevent SPN 4355?
Some tips to avoid SPN 4355:
- Maintain proper DEF levels, do not overfill
- Use high-quality DEF fluid from reputable suppliers
- Inspect wiring and connectors regularly for damage
- Address any DEF leaks or consumption issues promptly
What other components are related to line heater 2?
Line heater 2 is part of the DEF delivery system which also includes:
- The DEF tank and pickup
- DEF pump
- Other line heaters before and after line heater 2
- DEF injector
- Wiring harnesses connecting the heaters and sensors to the controller
Can low battery voltage cause SPN 4355?
Low voltage alone is unlikely to set SPN 4355 as it is an open circuit code. However, low system voltage can cause a variety of other issues that may lead to codes being set erroneously.
Will a failed line heater 2 always set SPN 4355?
Not necessarily. A failed heater element will likely set SPN 4355 if it results in an open circuit. But short circuits, for example, would set a different code.
Can extreme cold cause SPN 4355?
Extreme cold itself does not directly set SPN 4355. But it can lead to issues like frozen DEF that may cause the heater to work harder than normal, potentially leading to failure. Extreme temperatures can also make wiring more brittle.
Will adding an auxiliary DEF tank prevent SPN 4355?
Adding a second DEF tank does not directly prevent SPN 4355 as the code relates to the heater in the primary system. However, having additional DEF capacity can help avoid low DEF issues that may contribute to system strain.
Can SPN 4355 cause the vehicle to shut down?
SPN 4355 alone will not immediately shut down the vehicle. But if left unresolved, it can lead to SCR and DEF system issues that may eventually trigger a forced idle or low-speed derate.
Will clearing SPN 4355 resolve the issue?
No, simply clearing the code without performing repairs will not fix the problem. SPN 4355 will likely reappear until the root cause, such as a damaged heater element or wiring issue, is properly repaired.
How long does it take to repair SPN 4355?
Repair times vary depending on the specific issue and parts availability. A simple wiring repair or heater element replacement can often be done same-day. More complex issues involving the DEF controller may take longer, especially if the part is not in stock. Typical repair times are in the range of a few hours to a full day.
Hopefully this FAQ has provided a comprehensive overview of SPN 4355, its causes, symptoms, diagnosis, and repair. Let me know if you have any other questions!