SPN 4364 FMI 18 — SCR Conversion Efficiency — Below Threshold
The SCR catalyst isn't reducing NOx as much as it should — measured conversion efficiency is below the required threshold. The ECM flags it and, if it persists, stages a derate. The cause is usually upstream of the catalyst (DEF dosing/quality or sensors), not the catalyst itself.
Your SPN/FMI rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Insufficient or off-spec DEF dosing (clogged injector, poor DEF quality)
- Inlet/outlet NOx sensor drift or failure
- DEF dosing/supply pressure fault starving the SCR
- Aged, contaminated, or (rarely) tampered SCR catalyst
First moves (roadside)
- Verify DEF quality and level first — bad/contaminated DEF is the #1 cause of low SCR efficiency.
- Check for companion dosing (SPN 3361) and NOx-sensor codes that explain the low conversion.
- Inspect the dosing injector and lines for crystallization or restriction.
- Confirm both NOx sensors are reporting plausible values before condemning the catalyst.
Common misdiagnoses
- Replacing the expensive SCR catalyst when the real problem was DEF quality or a clogged doser.
- Swapping one NOx sensor without confirming which one drifted.
- Clearing the code without fixing dosing — efficiency stays low and it returns.
FAQ
The SCR catalyst isn't reducing NOx as much as it should — measured conversion efficiency is below the required threshold. The ECM flags it and, if it persists, stages a derate. The cause is usually upstream of the catalyst (DEF dosing/quality or sensors), not the catalyst itself.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Insufficient or off-spec DEF dosing (clogged injector, poor DEF quality). Other possibilities include Inlet/outlet NOx sensor drift or failure; DEF dosing/supply pressure fault starving the SCR; Aged, contaminated, or (rarely) tampered SCR catalyst.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.