Code Details
| Display code | SPN 3364 FMI 1 |
|---|---|
| SPN | 3364 |
| FMI | 1 |
| OEM code | None listed |
| Manufacturer | PACCAR |
| System | Aftertreatment �?DEF / SCR system |
| Component | DEF quality sensor / SCR system |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The PACCAR MX-11 uses an SCR aftertreatment system that depends on DEF with the correct urea concentration (32.5%) to convert NOx into nitrogen and water. The DEF quality sensor checks the actual concentration and rejects fluid that is too diluted or too concentrated. Common causes are using water, washer fluid, or low-quality DEF in the tank.
SPN 3364 is Aftertreatment 1 DEF Concentration in J1939. FMI 1 indicates the measured value is valid but below the acceptable minimum for SCR operation. The sensor on the MX-11 typically uses ultrasonic velocity measurement to determine urea concentration. PACCAR DAVIE can read the sensor value and verify whether the reading reflects actual poor-quality DEF or a sensor fault. If the DEF tank is drained and refilled with confirmed-quality DEF and the fault persists, the quality sensor may need replacement.
Common Symptoms
- DEF quality warning on the instrument cluster
- SCR system caution indicator active
- Inducement countdown visible in PACCAR DAVIE if the fault persists
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Incorrect fluid in DEF tank (water, washer fluid, or low-concentration product)
- Bulk DEF with incorrect concentration from a low-quality supplier
- Contamination from previous tank fill of a non-DEF fluid
- Faulty DEF quality sensor reading low when DEF is actually compliant
First Checks
- Drain the DEF tank completely and refill with fresh DEF from a reliable source �?use ISO 22241 certified product
- Connect PACCAR DAVIE and monitor the DEF concentration reading after refilling to confirm it returns to the acceptable range
- If the reading does not normalize after a confirmed-quality fill, test the DEF quality sensor using DAVIE
- Check the DEF tank for contamination residue if a non-DEF fluid was previously used
Can I Keep Driving?
After draining and refilling with quality DEF, the truck can continue operating. The inducement sequence will not activate immediately �?there is a period for the sensor to evaluate the new DEF. If inducement has already begun, the truck may derate until the quality is confirmed acceptable.
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 - SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
How is the PACCAR MX-11 different from the MX-13 in terms of DEF system design?
The MX-11 and MX-13 share a common DEF system architecture developed for PACCAR's European and North American emissions platforms. The DEF tank, pump module, dosing injector, and quality sensor operate on the same principles. The primary differences are in the aftertreatment sizing matched to the engine displacement and output. SPN 3364 FMI 1 diagnosis is essentially identical between the two engines.
Will SPN 3364 FMI 1 clear on its own after refilling the DEF tank on the PACCAR MX-11?
In most cases, yes. After refilling with compliant DEF, the quality sensor evaluates the new fluid over a drive cycle. If the concentration returns to the acceptable range, the fault becomes inactive. Depending on how long the poor-quality DEF was in the system, the sensor may need one or more drive cycles to re-evaluate and clear the code.
Can I add DEF concentrate or an additive to fix a low-concentration reading on the MX-11?
No. DEF must be ISO 22241 standard 32.5% aqueous urea solution. Adding concentrate, additives, or any other substance to try to adjust the concentration can further contaminate the system and may damage the dosing pump and injector. The correct approach is to drain and replace the entire tank contents with certified DEF.