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-13 aftertreatment system uses SCR technology to reduce NOx emissions by injecting DEF into the exhaust upstream of the SCR catalyst. The ACM measures the efficiency of NOx conversion by comparing upstream and downstream NOx sensor readings. SPN 4364 FMI 18 is set when the measured efficiency is below the moderately severe low threshold. DEF quality problems are the most frequent cause in the field — DEF that is diluted with water or contaminated will not produce the ammonia concentration required for effective NOx conversion in the catalyst. A clogged or failed DEF dosing injector, a degraded SCR catalyst, and inaccurate NOx sensor readings are additional causes. The ACM initiates a progressive inducement response that escalates to derates if the fault is not corrected.
SPN 4364 is Aftertreatment 1 SCR Conversion Efficiency; FMI 18 indicates valid data below normal at the moderately severe level. On the PACCAR MX-13, the ACM monitors SCR efficiency from upstream and downstream NOx sensor data and compares the result against the efficiency calibration. PACCAR ESA provides live data for both NOx sensors, DEF dosing system actuation, SCR temperature, and inducement timer status. The ESA DEF dosing injector test confirms injector operation. Refractometer testing of the DEF provides direct quality verification independent of the on-board sensor data.
Common Symptoms
- Check engine and aftertreatment warning lamps on
- DEF system or SCR efficiency warning on the driver display
- Engine derate if the inducement timer has advanced
- DEF consumption different from expected for the duty cycle
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF quality outside specification — diluted, contaminated, or degraded urea solution
- DEF dosing injector clogged or delivering insufficient DEF volume
- SCR catalyst degraded from oil contamination or age
- Upstream or downstream NOx sensor failure or drift
- DEF supply circuit fault limiting DEF flow to the injector
First Checks
- Test DEF concentration with a refractometer to verify 32.5% +/- 1.5% urea specification
- Connect PACCAR ESA and view live NOx sensor data at SCR inlet and outlet
- Run the ESA DEF dosing injector actuation test
- Inspect the DEF filter and supply line for restriction or crystallization
- Check inducement timer status in ESA to assess urgency of derate escalation
Can I Keep Driving?
The inducement timer may be progressing toward a derate. Address DEF quality and dosing faults promptly to prevent derate escalation.
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
What DEF concentration does the PACCAR MX-13 require?
The PACCAR MX-13 requires ISO 22241-compliant DEF with a urea concentration of 32.5% +/- 1.5%. Both higher and lower concentrations will reduce SCR conversion efficiency. Higher concentration can cause crystallization issues in the dosing system, while lower concentration does not generate enough ammonia for full NOx conversion across the catalyst. A refractometer that reads the DEF urea concentration directly is the most reliable field tool for verifying DEF quality.
Can a DEF tank heater failure indirectly cause SPN 4364 FMI 18 on the MX-13?
Yes. If the DEF tank heater fails in cold weather and the DEF freezes solid, the DEF dosing system cannot pump DEF to the injector, resulting in zero DEF delivery and extremely low SCR conversion efficiency. Once the DEF thaws, dosing may resume and efficiency can recover. However, if the tank heater fault is not corrected, the problem will recur whenever temperatures drop below the DEF freezing point (-11 degrees C / 12 degrees F). Check for DEF tank heater fault codes alongside SPN 4364 FMI 18 in winter-related cases.
Will SPN 4364 FMI 18 on the MX-13 resolve itself after adding fresh DEF?
If the fault was caused by out-of-spec DEF, draining the contaminated DEF, flushing the DEF system, and refilling with confirmed-quality DEF should resolve the efficiency issue. After refilling, the SCR system needs to operate for a period to recalculate the efficiency measurement with the new DEF. ESA can be used to monitor SCR efficiency data after the refill to confirm the conversion rate is returning to the normal range. The inducement timer may also need to be reset through ESA after the root cause is corrected.