SPN 4364 FMI 18 DTC Explained, Troubleshooting

SPN 4364 is a diagnostic trouble code for Aftertreatment 1 SCR Catalyst Outlet Gas.

FMI 18 – Scr Nox Conversion Efficiency Low

ACM – aftertreatment control module
EGR system, turbo, DEF injector, Doser pump


FMI codes associated with SPN 4364 include:

  • FMI 18: NOx conversion efficiency low – The SCR system is not effectively reducing NOx emissions, which could be caused by a variety of issues, such as low DEF quality, a malfunctioning DEF injector, or a damaged SCR catalyst.
  • FMI 1: NOx sensor issue – The NOx sensor may be providing incorrect or erratic readings, leading to incorrect NOx conversion efficiency calculations.


Troubleshooting tips:

  • Identify the issue: Note the specific fault code (SPN 4364) and any accompanying Failure Mode Identifier (FMI) codes that provide more detailed information about the issue. The problem is likely related to the SCR system’s NOx conversion efficiency.
  • Consult the manufacturer’s documentation: Refer to the engine manufacturer’s troubleshooting guide or service manual for information on the SCR system and any specific recommendations for addressing SPN 4364.
  • Perform a visual inspection: Check the SCR system components, such as the Diesel Exhaust Fluid (DEF) tank, DEF injector, SCR catalyst, and NOx sensors, for visible damage, leaks, or loose connections.
  • Verify DEF quality and level: Ensure that the DEF is of the correct specification and not contaminated. Also, make sure there is an adequate amount of DEF in the tank.
  • Check for additional fault codes: Use a diagnostic tool to scan the engine control module (ECM) for any additional fault codes that may be related to the SCR system, such as DEF injector or NOx sensor issues.
  • Test the NOx sensors: Verify that the NOx sensors are providing accurate readings by following the manufacturer’s testing procedures, which may involve comparing the sensor output with known good values or using a compatible diagnostic tool.
  • Check the DEF injector: Confirm that the DEF injector is functioning correctly and delivering the appropriate amount of DEF into the exhaust stream. Follow the manufacturer’s testing procedures or use a diagnostic tool to verify its operation.
  • Inspect the SCR catalyst: Examine the SCR catalyst for damage, clogging, or other signs of malfunction. A damaged or clogged catalyst may need to be replaced.
  • Verify repair: After identifying and addressing the cause of the problem, test the system to ensure that the SCR system is working correctly and that the SPN 4364 fault code has been resolved. Clear the fault codes with a diagnostic tool and run the engine to see if the code reappears.



An SPN code is a suspect parameter number. It is a diagnostic fault code. A FMI is a failure mode identifier that gives more details about error.