You are currently viewing SPN 4334 FMI [2-18] Troubleshooting Explained

SPN 4334 FMI [2-18] Troubleshooting Explained

SPN 4334 is a diagnostic trouble code for Aftertreatment 1 Diesel Exhaust Fluid Doser.

FMI 2 – Absolute Pressure Data Erratic, Intermittent Or Incorrect
FMI 3 – Absolute Pressure Voltage Above Normal, Or Shorted To High Source
FMI 4 – Absolute Pressure Voltage Below Normal, Or Shorted To Low Source
FMI 16 – Absolute Pressure Data Valid But Above Normal Operating Range – Moderately Severe
FMI 18 – Absolute Pressure Data Valid But Below Normal Operating Range – Moderately Severe

Troubleshoot: DEF sensor

  • Perform a visual inspection: Check the SCR system components, such as the DEF tank, DEF lines, DEF pressure sensor, and wiring for visible damage, leaks, or loose connections.
  • Check the DEF pressure sensor: Verify that the DEF pressure sensor is providing accurate readings by following the manufacturer’s testing procedures or using a compatible diagnostic tool.
  • Inspect wiring and connectors: Examine the wiring and connectors associated with the DEF pressure sensor for signs of damage, corrosion, or loose connections. Repair or replace any damaged components as necessary.
  • 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 pump or DEF injector issues.
  • Test the DEF pump: Confirm that the DEF pump is functioning correctly and delivering the appropriate pressure. Follow the manufacturer’s testing procedures or use a diagnostic tool to verify its operation.






I’ll never forget the day I was faced with the daunting SPN 4334 error. It was a typical Tuesday afternoon, and I was in the middle of a critical task at work when my computer screen froze, and that cryptic error code flashed before my eyes. The frustration was palpable; deadlines were looming, and here I was, staring down the barrel of a technical meltdown.

As I began to research, I learned that SPN stands for Suspect Parameter Number, a diagnostic code commonly used in the automotive industry, particularly for heavy-duty vehicles. It was odd seeing this in relation to my work computer, but I pressed on, determined to unravel the mystery. The error pointed to an issue with Diesel Exhaust Fluid (DEF) Pressure Sensor Circuit Voltage Below Normal. In the context of a vehicle, this would be a serious concern affecting emissions, but in my case, it was a clear sign that something was amiss with my machine’s hardware or software.