Primary fault reference: SPN 4364 FMI 18 — Full Fault Code Reference — This page covers the step-by-step repair workflow. For the complete fault definition, symptoms, causes, and derate risk, see the primary reference above.
Fix SPN 4364 FMI 18 — SCR Efficiency Below Threshold
Diagnose and repair low SCR conversion efficiency faults
Overview
SPN 4364 FMI 18 indicates the SCR (Selective Catalytic Reduction) catalyst is not converting NOx at the required efficiency. The ECM calculates SCR efficiency by comparing upstream and downstream NOx sensor readings. Efficiency below 70% triggers this fault and will eventually cause inducement.
Safety Warnings
- ⚠Allow exhaust system to cool before inspecting SCR components — temperatures can exceed 600°C during regen.
- ⚠Wear heat-resistant gloves when working near aftertreatment components.
- ⚠Do not inhale DEF vapors — work in a well-ventilated area.
Step-by-Step Repair
Read all active fault codes
Document all active and inactive fault codes. SPN 4364 FMI 18 is often accompanied by NOx sensor faults, DEF dosing faults, or DEF quality faults.
Check DEF quality and level
Verify DEF concentration is 32.5% ± 1.5%. Low-quality DEF is the most common cause of low SCR efficiency. Drain and refill if out of spec.
Monitor NOx sensor readings in live data
With engine at operating temperature, monitor upstream NOx (sensor 1) and downstream NOx (sensor 2) readings. Upstream should be 200–800 ppm at idle. Downstream should be 10–50% of upstream. If downstream equals upstream, SCR is not converting.
Verify DEF dosing is active
Monitor DEF dosing rate in live data during highway driving. If dosing rate is zero or very low, diagnose the DEF dosing unit, pump, and pressure sensor.
Test NOx sensors for accuracy
Compare NOx sensor readings to known reference values. A failed NOx sensor can cause false low efficiency readings. Swap upstream and downstream sensors to test — if efficiency improves, the swapped sensor was faulty.
Inspect SCR catalyst for damage
If DEF quality is good, dosing is active, and NOx sensors are accurate, the SCR catalyst may be damaged. Inspect for physical damage, substrate cracking, or contamination. A damaged SCR catalyst requires replacement.
Replace faulty components and verify
Replace faulty NOx sensors, dosing unit, or SCR catalyst as indicated by diagnosis. Clear fault codes and perform a drive cycle to verify SCR efficiency is above 70%.
Verification Checklist
- SCR efficiency above 70% in live data
- DEF dosing rate active during highway driving
- NOx sensor 1 and 2 readings within normal range
- No active fault codes after drive cycle
Related Fault Codes
- SPN 5246 FMI 31 — DEF Inducement
- SPN 4094 FMI 31 — DEF Quality
- SPN 2639 FMI 0 — NOx Sensor 1 Above Normal