SPN 5548 / Related — Cold Start Fuel Igniter Command / Relay Fault Class
Cold start fuel igniter faults follow the same electrical proof sequence as grid heaters: command, relay, resistance, current. Prove each gate before replacing the igniter.
Fault Overview
SPN 5548 and related parameters cover cold start fuel igniter command and relay status on platforms so equipped. Faults indicate command/feedback mismatch or circuit faults on the igniter system. The fuel igniter is used on selected heavy-duty and specialty platforms as a cold start aid in addition to or instead of grid/glow systems.
Engine & Platform Coverage
Symptoms
- •Cold start igniter fault active
- •Fuel igniter relay fault
- •Cold start aid igniter not responding
- •Hard start in extreme cold on equipped platforms
Possible Causes
- 1.Open or shorted igniter circuit
- 2.Failed relay
- 3.Command wire issues
- 4.Igniter element failure
- 5.Feedback circuit faults
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
4 steps locked behind Technician Mode- 1Step 1 — Confirm Command is Present: Verify the ECM is commanding the igniter via scan tool data. If the command is not present, diagnose the enable conditions before the circuit.
- 2Step 2 — Measure Relay Control and Load Side Voltages: Measure voltage at the relay control and load side in both commanded states. Confirm the relay is receiving the command and switching the load.
- 3Step 3 — Check Igniter Resistance: Measure igniter element resistance per OEM specification. An open element reads infinite resistance; a shorted element reads near 0 Ω.
- 4Step 4 — Measure Current Under Command: Command the igniter and measure current draw. Compare to OEM specification. No current with element connected confirms an open in the circuit path.
- 5Step 5 — Verify Feedback Status Matches Command: Confirm feedback or status signal matches the commanded state. A mismatch indicates a relay, feedback circuit, or monitoring issue.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Replace failed relay with correct OEM-specification unit.
- 2Replace failed igniter element.
- 3Repair open or shorted wiring.
- 4Repair feedback circuit fault.
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
Related Resources
Related Fault Codes
Related Systems
Related Sensor Locations
Common Symptoms
Related Articles
AI Diesel Tech — The Technician's Diagnostic Engine
AI Diesel Tech diagnostic logic is built by an ASE Certified Diesel Technician and engineered for professional shop use. Trusted by technicians nationwide — used in shops, fleets, and training programs.
Summary
Cold start fuel igniter faults follow the same electrical proof sequence as grid heaters: command, relay, resistance, current.