Detroit Diesel SPN 158 FMI 2: Meaning, Causes & Fix
Engine Start-Up Aid — data erratic or intermittent, intake air heater or start-up aid circuit not operating consistently
Reviewed by ASE Certified Mechanics · Last updated July 26, 2026
Quick Answer
SPN 158 FMI 2 = Engine Start-Up Aid Erratic
Severity: 🔵 INFO · System: Engine Start-Up Aid / Intake Grid Heater · ✅ Can continue with caution
Diagnostic Reference
| Field | Details |
|---|---|
| Code | SPN 158 FMI 2 |
| Protocol | J1939 SPN: 158 FMI: 2 |
| Component | Engine Start-Up Aid / Intake Grid Heater |
| Manufacturer | Detroit Diesel |
| Engine Series | DD13 |
| Severity | 🔵 INFO |
| SAE Reference | SAE J1939-73 Digital Annex — SPN 158, FMI 2 |
Possible Causes
- Intake grid heater element open circuit or degradation
- Grid heater relay contact wear or pitting
- Start-up aid temperature sensor fault
- ECM start-up aid driver circuit intermittent
- Wiring harness to grid heater damaged from heat
Diagnostic & Repair Procedure
- Step 1: Test grid heater element resistance with ohmmeter
- Step 2: Check grid heater relay operation with scan tool command
- Step 3: Inspect wiring to grid heater for heat damage
- Step 4: Verify start-up aid temperature sensor reading
- Step 5: Clear code and monitor operation at next cold start
Frequently Asked Questions
Is this a false alarm or a real problem?
CHECK SOON codes are rarely false alarms. The ECM has detected a parameter outside normal operating range. Sensor malfunctions can sometimes trigger false readings, but the majority of CHECK SOON codes indicate a developing issue that will worsen if left unaddressed. Use a diagnostic scan tool to verify sensor readings before replacing expensive parts.
Can I diagnose this without a scan tool?
Basic visual inspection can be performed without a scan tool — check wiring harnesses for chafing, connectors for corrosion, and related components for physical damage. However, to verify the fault, view live sensor data, and confirm the repair, a heavy-duty diagnostic scan tool (or a compatible OBD-II adapter with J1939/J1708 support) is strongly recommended for accurate diagnosis.
How soon should I schedule service?
Schedule service within the next 1–3 operating days. While the vehicle can continue operating normally, delaying repair increases the risk of the fault escalating to CHECK NEXT STOP severity. Additionally, some CHECK SOON conditions are precursors to emissions system faults that will trigger a mandatory derate if not addressed in a timely manner.
Estimated Repair Cost
Typical repair: $100–800 (Parts: $50–500 + Labor: 1–3 hours) · Costs vary by make/model and location
Related Fault Codes — DD13
SPN 4342 FMI 4
🟠 CHECK SOON
SPN 3226 FMI 9
🟠 CHECK SOON
SPN 102 FMI 4
SPN 524287 FMI 7
SPN 651 FMI 5
SPN 110 FMI 0
🔴 STOP ENGINE
Related Codes Across Systems
Codes from the same SPN range, manufacturer, or system that may also be relevant:
SPN 1569 FMI 31
🔴 STOP ENGINE
SPN 157 FMI 16
⚠️ WARNING
SPN 157 FMI 0
⚠️ WARNING
SPN 1761 FMI 18
SPN 520605 FMI 7
Associated Symptoms
References & Further Reading
- SAE J1939-73: Application Layer — Diagnostics. SAE International. Defines SPN 158 / FMI 2 fault code semantics for heavy-duty CAN networks. SAE J1939 Standard
- Detroit Diesel DD13 Service Manual: OEM diagnostic procedures for Engine Start-Up Aid / Intake Grid Heater faults. Consult the official Detroit Diesel service documentation for your specific engine serial number.
- TMC RP 1210: Recommended Practice for Windows-Based Vehicle Diagnostic Interface. Technology & Maintenance Council (TMC) of American Trucking Associations.