Selective Catalyst Reduction NOx Conversion Very low 4361/1 & 4364/18

One of the most common fault codes occurring in EPA10/GHG14 model units. This fault code appears when the ACM reads that NOx conversion is lower then spec. Based off the SCR inlet & outlet NOx sensors.

Conditions in which the code will appear:

  • During regen the DEF doser begins dosing
  • 1000-1500 rpm @15-100% load
  • SCR inlet temp less then 650C/ambient temp greater than 0C/coolant temp greater than 70C

This code will put the unit in a derate for at least one drive cycle but will not shut down the unit.


How to troubleshoot

  1. Verify no other emission related components were recently repaired. Perform a regen to verify code.
  2. Verify for any air system related fault codes, if any repair those first.
  3. Verify for any EGR related faults, if any repair those first.
  4. Verify for any DEF related faults, if any repair those first.
  5. Verify for any NOx sensor, DPF/DOC/SCR temp or pressure sensor fault. If any repair those first.
  6. Check DEF quality, verify there is no contamination. Use P/N DDE W060589001900 for more accurate readings. Replace DEF and flush system if any damage/contamination is present, then regen to verify repair.
  7. Using DDDL compare DEF pressure sensor to barometric pressure sensor reading. Make sure they are within 4.3PSI, if not replace DEF pressure sensor.
  8. Using DDDL perform DEF quantity test. Remove the DEF doser injector and verify the quantity of DEF fluid being sprayed. Spec is between 100-135ml. If not replace and regen to verify repair.
  9. Disconnect DEF doser injector connector and perform a Low temp regen using DDDL. Monitor inlet & outlet NOx sensor readings(PPM) make sure they are within 50ppm, if not replace the sensor with most inaccurate/erratic reading.
  10. After all basic checks have been performed. Regen the unit and monitor NOx efficiency and release unit if unit has over 70% efficiency.
  11. If unit does not pass a regen perform another regen and monitor SCR inlet & outlet temp sensors.


  • If the inlet SCR temp sensor reads 38C lower than the SCR outlet temp sensor, replace the One box and check for up stream failure
  • If temp sensors are within 38C, perform another low Temp regen. Monitor DPF/SCR temp sensors and replace sensors that have an erratic movement during the regen process.


9 thoughts on “Selective Catalyst Reduction NOx Conversion Very low 4361/1 & 4364/18

  1. Don Conner

    I have a dd15. When the engine is put under stress 25,000lbs or more in hilly terrain I get fault code 4364 (nox conv eff). Sometimes I will get fault 3364 (def eff) trk derates 25%. When these 2 faults come at same time I will have the yellow light in fuel gauge flash. If I am able to park sometimes it will let me manually regen. My question is this a pressure problem(emission control valve.

  2. Randy

    I have the one box system I have the following faults and when engine is under stress 15,000 lbs it blows out white smelly smoke from the tailpipe and it even goes into the cab ! Scr Inlet snsr , nox efficiency low,low temp regen,

  3. Mechanic Post author

    Try replacing the nox sensor and check for fuel and coolant leaks

  4. Brian

    Where are the locations of the NOX sensor??

  5. Mechanic Post author

    Look at the newest post for all sensor locations

  6. Enedina Tenpas

    Simply wanna say that this is handy, Thanks for taking your time to write this.



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