SPN 520326 FMI 4 is a Detroit DD Series diagnostic fault-code sheet that documents the Over Pressurized Diesel Exhaust Fluid (DEF) System condition and its factory troubleshooting sequence. The sheet lists the SPN and FMI identifiers, a short description of the condition, and the repair steps in the order Detroit recommends. It is a concise reference sheet, 2 pages long, aimed at technicians who read the code with a scan tool and need to know the correct next step.
SPN 520326 FMI 4 troubleshooting overview
This two-page sheet covers an over-pressurized Diesel Exhaust Fluid (DEF) system. After confirming no companion SPN 4334 or SPN 4374 faults, the technician inspects the DEF dosing unit and DEF pump harness, checks the DEF lines, purges the return line, then runs the SCR ADS self-check routine.
Official document details
| Brand | Detroit (Detroit Diesel) |
| Model coverage | DD Series (EPA 2007-Current) |
| System / group | Diagnostic Trouble Codes (DTCs) |
| SPN | 520326 |
| FMI | 4 — Over Pressurized Diesel Exhaust Fluid (DEF) System |
| Source | TruckFaultCodes.com fault-code reference (Copyright 2017) |
| Pages | 2 |
| File size | 0.2 MB |
| Format | |
| Language | English |
Document table of contents
- Fault code identification: SPN and FMI header
- Step 1–3: connect the diesel diagnostic tool and check for SPN 4334/4374 companion faults
- Step 4–5: inspect DEF dosing unit and pump harness, connectors and DEF lines
- Step 6: purge the DEF return line with clean shop air
- Step 7: SCR ADS self-check — DEF pump speed 750–1800 rpm, DEF pressure 9000–11000 mbar (130–160 psi)
Applicable truck models
This code sheet applies to Detroit DD Series engines (DD13, DD15 and DD16) built to GHG14 emissions specifications under the EPA 2007-Current platform, and Detroit cross-lists it under both the DD Series and Series 60 (DDEC VI, EPA 2007) diagnostic trees. For factory service literature and warranty details, refer to the official Detroit Diesel website.
Who needs this SPN 520326 FMI 4 manual
This reference is written for aftertreatment technicians servicing DEF pressure faults who need the harness inspection and SCR ADS self-check values. Keep the 520326 FMI 4 sheet on hand so the code can be diagnosed and verified before it is cleared.
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