SPN 1081 FMI 4 — Wait-to-Start Lamp Circuit Voltage Below Normal
Low voltage on the wait-to-start lamp is a short-to-ground or driver issue until wiring and bulb are proven good.
Fault Overview
FMI 4 indicates voltage below normal on the wait-to-start lamp control circuit, commonly a short to ground or failed low driver condition.
Engine & Platform Coverage
Symptoms
- •Wait-to-start lamp low voltage fault active
- •Glow light not working
- •Wait-to-start circuit short to ground
Possible Causes
- 1.Control wire shorted to ground
- 2.Failed bulb creating abnormal load
- 3.Cluster internal short
- 4.ECM driver failed low
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 — Measure Control Voltage On and Off: Measure voltage at the lamp control circuit in both commanded states. Low voltage in the commanded-off state confirms a short to ground is pulling the circuit down.
- 2Step 2 — Check for Short to Ground with Lamp Disconnected: Disconnect the lamp and measure resistance from the control wire to chassis ground. A short to ground will read near 0 Ω with the lamp disconnected, confirming the short is in the wiring rather than the lamp.
- 3Step 3 — Verify Bulb/LED Condition: Measure bulb resistance or LED condition. A failed bulb can create abnormal loading on the circuit.
- 4Step 4 — Inspect Harness Routing: Inspect the control wire routing near the cluster and firewall pass-through for pinching or damage. Water intrusion at the cluster is a common cause of short-to-ground conditions.
- 5Step 5 — Confirm ECM Output Capability: If wiring and bulb are proven good, confirm ECM output capability. An ECM driver failed low will pull the circuit to ground regardless of command state.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Repair shorted control wire identified by short-to-ground test.
- 2Replace failed bulb.
- 3Repair cluster circuit if internal short is confirmed.
- 4Repair ECM driver circuit only after external circuit is proven good.
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
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Summary
Low voltage on the wait-to-start lamp is a short-to-ground or driver issue until wiring and bulb are proven good.