SPN 158 FMI 2 is a Detroit™ ignition-switch diagnostic fault code, and this 2-page Detroit DD Series PDF documents the “Ignition Switch Not Plausible” condition in full. This code reports an implausible ignition-switch signal at the engine controllers. Diagnosis centres on ignition-supply voltage at the Motor Control Module 21-pin connector pin 7 and at the Common Powertrain Controller connector #2 pin 3, both of which should read greater than 11.5 volts. The summary below sets out the official causes, tools and troubleshooting order so a technician can verify the code before downloading the complete manual.
Official document details
| Brand | Detroit |
| Model coverage | DD Series (EPA 2007-Current) |
| System / Group | DD Series (EPA 2007-Current) / DTCs |
| Publication no. | Not printed on sheet (Detroit™ via TruckFaultCodes.com, © 2017) |
| Year / Revision | 2017 |
| Pages | 2 |
| File size | 0.3 MB |
| Format | |
| Language | English |
SPN 158 FMI 2 document contents
- Fault code identification (SPN and FMI)
- Active vs inactive fault check
- MCM 21-pin connector voltage check
- CPC connector #2 voltage check
- Repair guidance and support escalation
Applicable truck models
The procedure applies to Detroit DD Series engines across all model years that use a Motor Control Module and Common Powertrain Controller. Only a multimeter and the diagnostic tool are required. For factory service data, wiring diagrams and bulletins beyond this fault-code sheet, consult the official Detroit Diesel support site.
Who needs this SPN 158 FMI 2 manual
Technicians chasing an intermittent ignition-switch or hard-start complaint on a Detroit engine need this sheet to isolate low or missing key-on voltage before condemning a controller. It also documents when to call the Customer Support Center at 800-445-1980.
You confirm whether the code is active, then measure key-on voltage at MCM pin 7 and CPC #2 pin 3; a reading under 11.5 volts points to a supply-circuit repair, while good voltage with an active fault directs you to escalate.
The full 2-page PDF is available in the DiagTechPro manuals library; a free account is required to download it. Every document in the library is indexed by suspect parameter number and failure mode identifier, so you can cross-reference related Detroit DD Series fault codes without leaving the page.
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