SPN 4364 FMI 18
Derate riskSCR Conversion Efficiency — Below Normal, Moderately Severe
SAE J1939 · heavy duty · Freightliner, Volvo Trucks, PACCAR (Kenworth / Peterbilt), International, Mack, Cummins, Western Star
Power loss coming
The catalyst is not converting enough NOx to meet the certified standard. This is the fault that starts the inducement countdown — it does not clear itself and it does not wait.
DerateTODO(verify): confirm per-OEM schedule. A conversion efficiency fault is one of the faults that starts the regulator-mandated inducement clock, so it typically escalates to a torque derate and then to a speed limit if it stays active across drive cycles.
DrivingFor now, yes — but this fault begins the inducement sequence. Plan on getting it diagnosed within days, not weeks. The cost of ignoring it is not just the repair: it is a truck limited to walking pace somewhere you did not choose.
Typical repair cost
- Parts
- $3,500–$10,750
- Labor
- 2–8 hr × $125–$210/hr = $250–$1,680
Provisional figures — pending verification against our receipt dataset.
Labor rate: provisional national placeholder — TODO(verify), not yet from our collected dataset. How we build these ranges.
Need it fixed
Phone-verified shops, with the date we last called each one.
What SPN 4364 FMI 18 means
SPN 4364
SPN 4364 is aftertreatment 1 SCR conversion efficiency: the percentage of incoming NOx the catalyst removes, calculated by comparing the inlet sensor (SPN 3216) with the outlet sensor (SPN 3226). It is the single number the emissions certification is enforced against on the road.
FMI 18
FMI 18 means data valid but below normal operating range, moderately severe level. Efficiency is measurably below the calibration threshold but not yet at the most-severe FMI 1 level.
Read together, SPN 4364 FMI 18 is the ECM saying: scr conversion efficiency — below normal, moderately severe. The SPN identifies which parameter, the FMI identifies how it failed. Two codes sharing an SPN but carrying different FMIs are different faults with different repairs, which is why the SPN alone is never enough.
Common causes
- Insufficient or interrupted DEF dosing — injector crystallisation, pump pressure, or supply line restriction
- Out-of-spec or contaminated DEF
- SCR catalyst poisoned by oil, coolant, or sulphur, or thermally damaged
- Either NOx sensor drifting, which corrupts the efficiency calculation without either sensor setting its own codeTwo healthy-looking sensors can still produce a false efficiency fault if one has drifted within its own plausibility band.
- Exhaust leak between the dosing point and the outlet sensor
- Duty cycle that never brings the catalyst into its operating temperature window
- Decomposition tube or mixer fouled with urea deposits, so DEF never fully atomises
No frequency data yet. We have fewer than 5 verified repair receipts for SPN 4364 FMI 18, so we are not going to print percentages. Causes above are listed in no particular order. Had this fixed? Tell us what it turned out to be.
Technician note pending
A credentialed technician annotation for SPN 4364 FMI 18 has not been commissioned yet. Until it is, this page stays a draft and is excluded from our sitemap. We do not publish invented first-hand experience. How we verify content.
How a technician diagnoses it
- 1
Read all stored codes first. If an inlet or outlet NOx sensor code is also present, chase that first — the efficiency fault is downstream of it.
Tool: OEM service tool
- 2
Test the DEF with a refractometer and note the fill history.
Tool: DEF refractometer · Spec: 32.5% urea
- 3
Compare inlet and outlet NOx readings at key-on in ambient air. Any meaningful split means one sensor is drifting and the efficiency figure cannot be trusted.
Tool: OEM service tool
- 4
Run the dosing self-test. Watch commanded versus delivered quantity and rail pressure, not just pass or fail.
Tool: OEM service tool
- 5
Remove and inspect the DEF injector and the decomposition tube. Urea crystal build-up in the mixer is common and visible.
Tool: Hand tools, borescope
- 6
Check exhaust temperature history over the last several hundred hours. Extended idle keeps the catalyst cold and will produce this fault on healthy hardware.
- 7
Pressure test for exhaust leaks between the dosing point and the outlet sensor.
- 8
Treat catalyst replacement as the last step, after dosing, fluid, sensors, deposits, and leaks are all cleared. It is the most expensive part in the system and the one most often replaced unnecessarily.
Parts and labour
| Part | Part number | Parts cost |
|---|---|---|
| DEF dosing injector | TODO(verify) | $250–$750provisional |
| NOx sensor (inlet or outlet) with harness | TODO(verify) | $350–$900provisional |
| Decomposition tube / mixer assembly | TODO(verify) | $400–$1,600provisional |
| SCR catalyst assembly | TODO(verify) | $2,500–$7,500provisional |
| Labour | 2–8 hr | $250–$1,680 |
Labour priced at provisional national placeholder — TODO(verify), not yet from our collected dataset. How we build these numbers.
Can I keep driving?
For now, yes — but this fault begins the inducement sequence. Plan on getting it diagnosed within days, not weeks. The cost of ignoring it is not just the repair: it is a truck limited to walking pace somewhere you did not choose.
TODO(verify): confirm per-OEM schedule. A conversion efficiency fault is one of the faults that starts the regulator-mandated inducement clock, so it typically escalates to a torque derate and then to a speed limit if it stays active across drive cycles.
Find a shop that handles this
We call every shop before it appears here, and publish the date.
Sample records only — seeded to demonstrate the directory. None of these numbers has been called.
Memphis, TN · 24/7 · mobile service
Indianapolis, IN · Mon-Fri 6am-6pm · mobile service
Laredo, TX · Mon-Sat 6am-9pm · mobile service
Phoenix, AZ · 24/7 · mobile service
Codes that usually show up with this one
- SPN 5246 FMI 0SCR Operator Inducement — Most Severe Level ActiveOut of service
- SPN 3216 FMI 16SCR Inlet NOx Sensor — Reading Above Normal RangeCaution
- SPN 3226 FMI 16SCR Outlet NOx Sensor — Reading Above Normal RangeCaution
- SPN 3364 FMI 2DEF Quality / Concentration — Data Erratic or IncorrectDerate risk
Had this repaired? Tell us what you paid.
Every price range on this site comes from real invoices. Two minutes of your time makes the next driver's estimate better — and you get to see the aggregate for SPN 4364 FMI 18 once it publishes.
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