Code Details
| Display code | SPN 4364 FMI 18 |
|---|---|
| SPN | 4364 |
| FMI | 18 |
| OEM code | None listed |
| Manufacturer | PACCAR |
| System | Aftertreatment — SCR |
| Component | SCR catalyst / DEF dosing system / NOx sensors / DEF quality |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-12 |
Plain-English Meaning
The PACCAR MX-11 SCR system converts NOx in the exhaust using diesel exhaust fluid injected upstream of the SCR catalyst. The ACM compares NOx levels upstream and downstream of the catalyst to calculate conversion efficiency. When efficiency falls below the moderately severe threshold, SPN 4364 FMI 18 is set. On the MX-11, DEF quality issues, dosing injector faults, and SCR catalyst degradation are the most common causes. The ACM initiates an inducement sequence that escalates to derates if the efficiency fault is not corrected. Verifying DEF quality and the DEF dosing system function are the priority diagnostic steps before investigating the SCR catalyst or NOx sensors.
SPN 4364 is Aftertreatment 1 SCR Conversion Efficiency; FMI 18 indicates valid data below the moderately severe low threshold. On the PACCAR MX-11, the ACM monitors SCR efficiency from upstream and downstream NOx sensor readings. PACCAR ESA provides live data for both NOx sensors, DEF dosing actuation data, SCR temperature, and the inducement timer. The ESA DEF dosing injector actuation test confirms whether the injector opens and closes on command and that DEF is being injected into the exhaust. Refractometer DEF testing provides a direct quality verification that does not depend on the on-board DEF quality sensor reading.
Common Symptoms
- Check engine and aftertreatment warning lamps on
- SCR efficiency or DEF system warning message
- Engine derate if the inducement sequence has advanced
- DEF consumption change — higher or lower than expected
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF quality below specification
- DEF dosing injector clogged or not delivering correct DEF volume
- SCR catalyst degraded by oil contamination or age
- NOx sensor failure or calibration drift
- DEF supply circuit fault limiting DEF delivery
First Checks
- Refractometer test of DEF sample — confirm 32.5% +/- 1.5% urea concentration
- Connect PACCAR ESA — check live NOx sensor data at both upstream and downstream positions
- Run DEF dosing injector actuation test in ESA
- Inspect DEF filter and supply lines for restriction or crystallization
- Review inducement timer status in ESA
Can I Keep Driving?
The MX-11 inducement timer may be progressing. Correct DEF quality and dosing faults promptly to prevent escalation to more severe derates.
Related Codes
Related Lookup Pages
Sources
- PACCAR / Kenworth / Peterbilt Service Support — MX Engine Resources PACCAR Inc. · oem · accessed 2026-06-11 · confidence medium
Source: PACCAR Inc., PACCAR / Kenworth / Peterbilt Service Support — MX Engine Resources. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - PACCAR Engine Technical Services PACCAR Inc. · oem · accessed 2026-06-10 · confidence medium
Source: PACCAR Inc., PACCAR Engine Technical Services. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
Can mixing DEF brands cause SPN 4364 FMI 18 on the PACCAR MX-11?
Mixing different brands of ISO 22241-compliant DEF should not cause SPN 4364 FMI 18, since all compliant DEF products must meet the same urea concentration and purity specification. The concern arises when DEF from a non-compliant or unknown source is mixed with compliant DEF, diluting the urea concentration below the acceptable range. If DEF quality is suspected after mixing sources, test the tank contents with a refractometer to confirm the concentration is within the 32.5% +/- 1.5% range.
How long does the MX-11 SCR inducement derate last after SPN 4364 FMI 18 is corrected?
After correcting the root cause of SPN 4364 FMI 18, the inducement timer does not immediately reset on its own in all cases. PACCAR ESA may need to be used to perform an inducement reset procedure after confirming the SCR efficiency has returned to the acceptable range. The specific reset procedure depends on the GHG17 calibration and the current inducement level. Consult the applicable PACCAR service documentation or a PACCAR dealer service department for the current reset procedure.
Will replacing the DEF tank with fresh DEF fix SPN 4364 FMI 18 on the MX-11 if it was a quality issue?
Draining the contaminated DEF, flushing the DEF system, and refilling with fresh ISO 22241-compliant DEF should resolve the efficiency fault if DEF quality was the root cause. The SCR system efficiency calculation may need a period of operation after the refill before INSITE or ESA shows the efficiency returning to the normal range. Monitor SCR efficiency data in ESA after the refill to confirm recovery before performing an inducement reset.