SPN 91 FMI 4 is the Detroit fault code for a circuit failed low on the DD Series accelerator pedal circuit. This 2-page reference from the DiagTechPro library reproduces the official Detroit troubleshooting steps so a technician can work the fault in the correct order. It is a factory-style diagnostic guide, not a repair shortcut.
Official document details
| Brand | Detroit (Detroit Diesel) |
| Model coverage | DD13, DD15, DD16 – DD Series (EPA 2007–current) |
| System / Group | DD Series (EPA 2007-Current) / DTCs |
| Suspect Parameter Number | 91 |
| Failure Mode Identifier | 4 |
| Description | Circuit Failed Low |
| Coverage / Revision | ALL YEARS |
| Pages | 2 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Fault code identification and description
- Accelerator Pedal (AP) harness voltage checks
- Common Powertrain Controller (CPC) circuit checks
- AP resistance and continuity checks
- Clearing codes and verifying the repair
SPN 91 FMI 4 diagnostic overview
SPN 91 FMI 4 covers an accelerator pedal circuit-failed-low fault. The ten-step procedure measures supply voltage, traces open circuits to the Common Powertrain Controller (CPC) #1 connector, then checks a series of resistance values for shorts between pins and to ground before the pedal is replaced. Each step is written as a yes or no decision, so the result of one measurement points directly to the next check or to the final repair action.
Applicable truck models and engines
It applies to Detroit DD13, DD15 and DD16 engines in the DD Series (EPA 2007 to current) family, and the library also carries the matching guide for the Series 60 (DDEC VI) platform. For factory support tools and warranty information, see Demand Detroit official resources.
Who needs this 91 FMI 4 manual
This document is written for Detroit DD Series technicians who need the full short-to-ground and short-between-wires checks that separate a wiring fault from a failed accelerator pedal.
The full PDF is available in the DiagTechPro manuals library; a free account is required to download it, view every page, and print the decision steps for use at the workshop bench.
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