SPN 4364: Aftertreatment SCR Conversion Efficiency
What SPN 4364 Is
SPN 4364 is Aftertreatment 1 SCR Catalyst Conversion Efficiency. The ECM subtracts outlet NOx (3226) from inlet NOx (3216). When conversion stays below the calibration threshold, 4364 sets. This is the code shops used to file under 3216 in error.
It is a system fault, not a part number. DEF quality, dosing, NOx sensor lies, exhaust leaks, sulfur-poisoned catalyst, and a dead brick all look like 4364 on the screen.
Common FMI Combinations
| FMI | Meaning | Shop note |
|---|---|---|
| 18 | Below normal, moderately severe | Most common. Investigate, truck may still roll. |
| 1 | Below normal, most severe | Often with derate |
| 31 | Condition exists | Sustained failure; watch 5246 |
Do Not Start With the Catalyst
- Refractometer: 31–34% urea. Drain and flush if it is diesel, water, or exhausted fluid. See SPN 3364.
- DEF quantity / doser test in OEM software. Clogged doser and crystal in the mixer fake a dead SCR.
- Graph both NOx sensors. If outlet reads as high as inlet with a known-good dose, then consider the brick — after leaks are ruled out.
- Only then SCR catalyst. On Cummins X15, high-mileage bricks do fail; they are still not the first parts.
Unresolved 4364 feeds derate 1569 and inducement 5246. We run the SCR tests remotely with a Nexiq USB-Link 3 and a diagnostic laptop (INSITE SCR test, DDDL aftertreatment monitor, ESA).
Frequently Asked Questions
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