SPN 520323 FMI 4 is a Detroit DD Series diagnostic fault-code sheet that documents the High Side Digital Output Circuit Failed Low 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 520323 FMI 4 troubleshooting overview
This two-page sheet covers a high-side digital output circuit failed low on the DEF dosing unit, monitored on DEF dosing unit voltage with a 25% derate. The technician inspects the dosing unit connector for bent, spread or corroded pins, disconnects the ACM2.1 120-pin connector, and measures resistance between pins to locate and repair a short.
Official document details
| Brand | Detroit (Detroit Diesel) |
| Model coverage | DD Series (EPA 2007-Current) |
| System / group | Diagnostic Trouble Codes (DTCs) |
| SPN | 520323 |
| FMI | 4 — High Side Digital Output Circuit Failed Low |
| Source | TruckFaultCodes.com fault-code reference (Copyright 2017) |
| Pages | 2 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Fault code identification and description (DEF dosing unit voltage, MIL/CEL, 25% derate)
- Step 1–3: disconnect and inspect the DEF dosing unit harness connector
- Step 4: disconnect the ACM2.1 120-pin connector
- Step 5–6: measure resistance between pins 2 and 7 and to ground, then repair the short
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 520323 FMI 4 manual
This reference is written for electrical technicians tracing DEF dosing circuit faults who need the connector inspection and pin resistance measurements. Keep the 520323 FMI 4 sheet on hand so the code can be diagnosed and verified before it is cleared.
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