SPN 4364 — SCR Catalyst Conversion Efficiency, Aftertreatment 1
Safe to driveTo a shop, without delay
Updated: 2026-11-14
J1939 code SPN 4364. Below — how the fault is worded at the factory by Cummins ISB and KAMAZ-5490, and what the second half of the code (FMI) means in the standard.
How to check it yourself
SCR catalyst (SCR 1) in the exhaust: urea from the doser turns NOx into nitrogen and water. The ECU compares NOx before and after the catalyst. The code is about exceeded emissions, not necessarily a broken catalyst.
What it should read
| NOx after the catalyst, first threshold | per the manufacturer's manual |
| NOx after the catalyst, second threshold | per the manufacturer's manual |
Check order
- FMI 15 (above normal, low urgency) and FMI 16 (above normal, medium urgency). On KAMAZ-5490 these are MR2 12971 and MR2 12972. First read the NOx values before and after the catalyst with the engine warm.
- Check the AdBlue: quality, shelf life, level in the tank. Poor-quality urea lowers NOx conversion, and emissions go above the threshold.
- Check the AdBlue doser and injector: no dose means NOx rises, and the code appears at the catalyst.
- Inspect the exhaust before the catalyst and the sensor: air leaks give a low NOx reading or, the other way round, a false value.
- After repair, clear the code and do a test drive. If the code comes back after warm-up, keep looking instead of replacing the sensor.
Common causes — from most to least frequent
- AdBlue of poor quality or out of date.
- Clogged or faulty AdBlue doser module.
- Catalyst wear or blockage.
- Faulty NOx sensor after the catalyst.
- Exhaust leak before the sensor.
Cummins ISB 6.7 engine
| FMI 18 | SCR catalyst conversion efficiency low, moderately severe |
| FMI 31 | SCR conversion efficiency, condition exists |
KAMAZ-5490 (Mercedes, K-line)
FMI is per the block maker's own table, not the J1939 table below.
| FMI 15 | NOx emissions: level 1 exceeded. K-line code on KamAZ-5490: MR2 12971. |
| FMI 16 | NOx emissions: level 2 exceeded. K-line code on KamAZ-5490: MR2 12972. |
What FMI means in J1939
| SPN 4364 FMI 184364/18 · 4364.18 | below normal — moderate severity |
| SPN 4364 FMI 314364/31 · 4364.31 | condition exists (see manufacturer description) |
Can I keep driving
You can drive to a shop, but do not put it off. If a power or speed limit has kicked in — stop and sort it out.
What happens if you keep driving
The unit uses two NOx sensors, before the SCR catalyst and after it, to work out how much NOx the catalyst removes. If it is too little, the unit sets this code.
A power or speed limit may switch on, as the emissions rules require; the conditions depend on the manufacturer. Plan the repair now, not after the limit has already kicked in.
What to check on the spot
Check the quality, shelf life and level of the AdBlue, then the doser and any exhaust leak before the NOx sensors. If the AdBlue is fine, you need a scan tool: look at the NOx readings before and after the catalyst.
Nearby: AdBlue / exhaust
- SPN 131Exhaust back pressure
- SPN 1761AdBlue (DEF) tank level
- SPN 3050Catalyst System Monitor 1
- SPN 3364Aftertreatment 1 AdBlue Tank Quality
- SPN 4334AdBlue Doser Absolute Pressure, Aftertreatment 1
Makes with this code
Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.