SPN 158 FMI 2 – Ignition Switch Not Plausible Fault Code (Symptoms, Causes & Fix)
The SPN 158 FMI 2 fault code is a diesel engine diagnostic trouble code indicating an 'Ignition Switch Not Plausible' condition. This code suggests that the Engine Control Module (ECM) is receiving an inconsistent or unexpected signal from the ignition switch, which could affect the truck's starting and electrical systems.
What SPN 158 FMI 2 Means
When the SPN 158 FMI 2 fault code is active, it means the ECM has detected a discrepancy in the ignition switch signal. The ignition switch is responsible for sending the correct signal to the ECM to initiate the engine start sequence. If the signal is not plausible, it could be due to a faulty switch, wiring issues, or other electrical problems.
Common Causes of This Fault Code
- Faulty ignition switch
- Wiring harness issues such as shorts or opens
- Corroded or loose connectors
- Faulty ignition relay
- Battery voltage issues
Symptoms Drivers May Notice
Drivers may experience difficulty starting the engine, intermittent starting issues, or complete failure to start. Additionally, there may be dashboard warning lights related to the ignition system.
How Technicians Diagnose This Fault
Technicians begin by performing a visual inspection of the ignition switch and related wiring for any visible damage or corrosion. Next, they use a multimeter to check for continuity and proper voltage at the ignition switch. The ignition relay is also inspected and tested for proper operation. Battery voltage and connections are checked to ensure they are within specifications. Finally, diagnostic software is used to monitor the ignition switch status and compare it with expected values.
Common Misdiagnosis
Technicians often mistakenly replace the ignition switch without verifying the integrity of the wiring. Overlooking battery voltage issues is another common error. Additionally, some may assume a faulty ECM without thorough testing, leading to unnecessary part replacements.
Can You Keep Driving?
While the truck may still operate with this fault code active, it is not advisable to continue driving without addressing the issue. Ignition system problems can lead to engine derate or protection mode, potentially leaving the vehicle inoperable.
Mechanic's Diagnostic Tip
Always verify the integrity of the wiring harness and connectors before replacing the ignition switch. Many issues stem from simple wiring faults rather than component failures.
Frequently Asked Questions
Q: What does SPN 158 FMI 2 indicate?
A: It indicates an 'Ignition Switch Not Plausible' condition, suggesting an issue with the ignition switch signal.
Q: Can a faulty battery cause this code?
A: Yes, battery voltage issues can contribute to this fault code, so checking the battery is essential.
Q: Is it safe to drive with this fault code?
A: It's not recommended to drive with this fault active as it can lead to engine derate or protection mode.
