Lift Pump / Fuel Supply Pump Enable Circuit Voltage Below Normal / Open
Open or low voltage on the lift pump circuit is diagnosed by confirming voltage at the pump under command and checking voltage drop before pump replacement.
Fault Overview
Voltage below normal or open condition on the lift pump enable circuit. The ECM cannot energize the pump or detects the circuit is not responding.
Engine & Platform Coverage
Symptoms
- •Lift pump open circuit code
- •Fuel supply pump no power
- •Lift pump enable low voltage
- •Low pressure pump inoperative
- •Hard start no lift pump
Possible Causes
- 1.Open control wire
- 2.Failed open relay
- 3.Blown fuse
- 4.Open power feed to the pump
- 5.Failed pump motor
- 6.Poor ground
- 7.ECM driver failed open (rare)
Wiring & Circuit Notes
Check fuse, relay socket, power feed cables, and ground path. Inspect for corrosion at the pump connector, especially on frame-mounted pumps. Corroded ground and power connections are frequent causes.
DVOM Specs & Sensor Tests
Voltage at the pump power pin when commanded on. Control circuit continuity and voltage. Pump motor resistance. Current draw when running — typical 4–10 A depending on design.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Step 1 — Check Related Fuses: Check all fuses related to the lift pump circuit. A blown fuse is the simplest and most common cause.
- 2Step 2 — Measure Voltage at Pump When Commanded On: Measure voltage at the pump power pin when commanded on. No voltage confirms the circuit is open upstream.
- 3Step 3 — Verify Control Signal at Relay: Verify the control signal from the ECM is reaching the relay coil.
- 4Step 4 — Test Relay Operation: Test relay operation by substituting a known-good relay or jumping the relay socket.
- 5Step 5 — Measure Pump Motor Resistance and Ground: Measure pump motor resistance and verify ground integrity at the pump connector.
- 6Step 6 — Voltage-Drop Test Under Load: Perform voltage-drop test on power and ground circuits under load. Corroded ground and power connections at frame-mounted pumps are frequent.
- 7Step 7 — Confirm ECM Command: Confirm ECM command with scan data. Only after external circuit is proven open or good should ECM be suspected.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Replace fuse or relay $20–60; repair open wire or corroded connection $50–150; replace failed pump $150–350; correct ground $30–80. Total typical $80–450.
Start Diagnosing This Fault Now
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Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.