SPN 4364 FMI 1 — Aftertreatment 1 Diesel Exhaust Fluid Quality - Data Valid But Below Normal Operating Range, Most Severe Level (DEF quality/concentration incorrect)
What it means: Aftertreatment 1 Diesel Exhaust Fluid Quality - Data Valid But Below Normal Operating Range, Most Severe Level (DEF quality/concentration incorrect).
How serious / can you keep driving: Critical. Stop as soon as it's safe.
Check first: 1) Pull a DEF sample from the fill neck and check concentration with a refractometer - correct spec is 32.5 percent urea plus/minus 0.5 percent. 2) Inspect the DEF quality sensor connector (integrated in the tank sending unit
Don't risk it — look up listed providers near you and give them this SPN/FMI when you call.
Listed providers nearest Billings, MT
Closest first. Tell them SPN 4364 FMI 1 and what the truck is doing. RoadService.app lists these providers and does not employ them or carry out the repair.
- 0.0 mi away — based in Billings, MT · (406) 245-8985
- 0.0 mi away — based in Billings, MT · (406) 252-8413
- 0.0 mi away — based in Billings, MT · (406) 780-3711
Applies to: Cummins Isx15 · Freightliner Cascadia · International ProStar (ISX15 2013-2019) · Kenworth T680 · Peterbilt 579
What SPN 4364 FMI 1 means
Aftertreatment 1 Diesel Exhaust Fluid Quality - Data Valid But Below Normal Operating Range, Most Severe Level (DEF quality/concentration incorrect).
How serious is it?
Critical. Stop as soon as it's safe.
Most likely causes
- DEF diluted with water or contaminated with diesel/other fluid
- often from a bad batch
- an unsealed cap
- or someone topping off with the wrong fluid
- DEF quality sensor failure or corroded connector
- DEF that has frozen/thawed and crystallized repeatedly
- expired/degraded DEF from long storage in heat
First moves (roadside)
- 1) Pull a DEF sample from the fill neck and check concentration with a refractometer - correct spec is 32.5 percent urea plus/minus 0.5 percent. 2) Inspect the DEF quality sensor connector (integrated in the tank sending unit
- 6-pin Deutsch under the tank) for corrosion or moisture intrusion. 3) Smell/look for a diesel sheen or crystallization in the tank confirming contamination versus a bad sensor
FAQ
Aftertreatment 1 Diesel Exhaust Fluid Quality - Data Valid But Below Normal Operating Range, Most Severe Level (DEF quality/concentration incorrect).
Critical — Stop as soon as it's safe.
The most common cause is: DEF diluted with water or contaminated with diesel/other fluid. Other possibilities include often from a bad batch; an unsealed cap; or someone topping off with the wrong fluid; DEF quality sensor failure or corroded connector; DEF that has frozen/thawed and crystallized repeatedly; expired/degraded DEF from long storage in heat.
Stop as soon as it's safe. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Related codes
Codes that commonly appear alongside SPN 4364 FMI 1, or share the same subsystem.
- SCR Conversion Efficiency — Below Threshold (SPN 4364 FMI 18)
- SCR Operator Inducement — Severe Derate Active (SPN 5246 FMI 0)
- Aftertreatment 1 Diesel Particulate Filter Differential Pressure - Data Valid But Above Normal Operating Range, Most Severe Level (DPF severely restricted / plugged) (SPN 3251 FMI 0)
- DPF Differential Pressure — Data Erratic (SPN 3251 FMI 2)
- DEF Dosing Unit — Not Responding Properly (SPN 3361 FMI 7)
- DEF Quality — Out of Spec (SPN 4094 FMI 31)
- EGR Temperature — Too High (SPN 412 FMI 0)
- SCR NOx Catalyst Efficiency Below Threshold — Bank 1 (P20EE)
Same symptom: DEF / SCR fault
Other codes in the DEF and SCR chain — the ones that end in an inducement countdown.
- Reductant System Performance Bank 1 (P204F) · Aftertreatment & Emissions
- Reductant Quality Deviation (P20EA) · Aftertreatment & Emissions
- DEF tank level low (SPN 1761 FMI 18) · Aftertreatment & Emissions
- DEF tank temp sensor — voltage high (SPN 3031 FMI 3) · Aftertreatment & Emissions
- Upstream NOx sensor — internal failure reported (SPN 3216 FMI 12) · Aftertreatment & Emissions
- SCR inlet NOx sensor — erratic (SPN 3216 FMI 2) · Aftertreatment & Emissions
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.