What Is SPN 4364 FMI 18?

SPN 4364 is Aftertreatment SCR Conversion Efficiency. FMI 18 means the efficiency is below normal (moderately severe). The ECM is measuring the difference in NOx levels before and after the SCR catalyst — and the catalyst is not converting enough NOx. This fault leads to SPN 5246 (inducement) if not resolved.

Root Causes in Order of Frequency

1. 1

   Bad DEF quality (40%) — test with refractometer before anything else

2. 2

   Failed upstream NOx sensor (20%) — bad sensor gives false high NOx reading

3. 3

   Clogged DEF dosing injector (15%) — insufficient DEF reaching the catalyst

4. 4

   Failed downstream NOx sensor (10%) — bad sensor gives false high NOx reading after catalyst

5. 5

   DEF pump low pressure (8%) — insufficient DEF flow

6. 6

   SCR catalyst contamination or failure (5%) — last resort diagnosis

7. 7

   Exhaust leak upstream of SCR (2%) — dilutes exhaust, reduces apparent efficiency

Step-by-Step Diagnostic Procedure

Follow in order. Do not skip steps.

1. 1

   Test DEF quality with a refractometer. Spec is 32.5% urea (±1%). Replace DEF if out of spec and retest.

2. 2

   Check for active NOx sensor faults (SPN 3226, 3227). If present, diagnose those first.

3. 3

   With a scan tool, monitor upstream and downstream NOx sensor values at operating temperature. Both sensors should read differently — upstream higher than downstream. If both read the same, one sensor is failed.

4. 4

   Inspect the DEF dosing injector. Remove and inspect for crystallization. Test injector flow if possible.

5. 5

   Check DEF pump pressure. Spec varies by OEM — typically 8–10 bar. Low pressure indicates pump wear or supply line restriction.

6. 6

   Perform a forced regen and monitor SCR efficiency during and after regen. Efficiency should improve if DEF quality and dosing are correct.

7. 7

   If efficiency remains below threshold after all above checks, the SCR catalyst is likely contaminated or failed.

NOx Sensor Testing Procedure

If you suspect a NOx sensor fault:

1. 1

   Check sensor connector for corrosion, bent pins, or moisture intrusion.

2. 2

   Verify sensor heater circuit — measure resistance between heater pins. Spec is typically 2–10 ohms. Open circuit means failed heater.

3. 3

   Monitor sensor output with a scan tool at idle and under load. A failed sensor often reads 0 ppm or a fixed value that doesn't change with engine load.

4. 4

   Compare upstream and downstream readings. After a successful regen, downstream NOx should be significantly lower than upstream.