SPN 520327 FMI 3 is a Detroit DD Series diagnostic fault-code sheet that documents the Under Pressurized 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, 3 pages long, aimed at technicians who read the code with a scan tool and need to know the correct next step.
SPN 520327 FMI 3 troubleshooting overview
This three-page sheet covers an under pressurized DEF system, monitored on DEF pump speed versus DEF pressure with the CEL illuminated. The procedure inspects the DEF dosing unit and pump harness, checks the DEF lines, purges the suction and pressure lines, inspects the suction, dosing unit inlet and tank supply filters, and replaces the pressure relief valve if the SCR ADS self-check still fails.
Official document details
| Brand | Detroit (Detroit Diesel) |
| Model coverage | DD Series (EPA 2007-Current) |
| System / group | Diagnostic Trouble Codes (DTCs) |
| SPN | 520327 |
| FMI | 3 — Under Pressurized DEF System |
| Source | TruckFaultCodes.com fault-code reference (Copyright 2017) |
| Pages | 3 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Fault code identification and description (DEF pump speed vs. DEF pressure, CEL)
- Step 1–5: check companion SPN 4334/4374 faults, inspect DEF dosing unit and pump harness and DEF lines
- Step 6–9: purge lines and inspect the suction, dosing unit inlet and tank supply filters
- Step 10–11: SCR ADS self-check and pressure relief valve (PRV) replacement
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 520327 FMI 3 manual
This reference is written for aftertreatment technicians diagnosing low DEF pressure who need the harness, filter and PRV replacement sequence. Keep the 520327 FMI 3 sheet on hand so the code can be diagnosed and verified before it is cleared.
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