Lift Pump / Fuel Supply Pump Enable Circuit Voltage Above Normal
High voltage on the lift pump enable circuit is a short-to-power or relay issue until state testing proves otherwise.
Fault Overview
Voltage above normal on the lift pump (low-pressure supply pump) enable or control circuit. The ECM detects high voltage inconsistent with the commanded off or on state.
Engine & Platform Coverage
Symptoms
- •Lift pump high voltage code
- •Fuel supply pump circuit fault
- •Low pressure pump enable fault
- •Lift pump relay voltage code
Possible Causes
- 1.Control wire shorted to battery voltage
- 2.Failed relay contacts stuck closed
- 3.Incorrect relay installed
- 4.Wiring damage near heat sources
- 5.Aftermarket pump wiring introducing a short
- 6.ECM driver fault (rare)
Wiring & Circuit Notes
Inspect lift pump relay socket, control wire, and power feed. Check for aftermarket pump wiring that may have introduced a short. Relay failures and aftermarket lift pump installations are common sources of circuit voltage faults.
DVOM Specs & Sensor Tests
Control circuit voltage in commanded off and on states. Relay coil resistance per OEM specification. Power side voltage at the pump when enabled. Current draw of the pump motor.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Step 1 — Identify Driver Type: Identify whether the driver is high-side or low-side. This determines expected voltage states in commanded on and off conditions.
- 2Step 2 — Measure Control Voltage in All States: Measure control voltage key-off, key-on, and during commanded pump operation. Compare to expected states for the driver type.
- 3Step 3 — Check Relay Coil and Contacts: Check relay coil resistance and contact operation. Stuck-closed contacts produce high voltage on the control circuit.
- 4Step 4 — Inspect for Short to Power: Inspect for short to battery voltage on the control wire along the harness route.
- 5Step 5 — Verify Power and Ground to Pump: Verify power and ground to the pump. Measure pump current draw.
- 6Step 6 — Isolate Aftermarket Devices: Identify and isolate any aftermarket pump wiring that may have introduced a short or incorrect relay.
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
Repair shorted control wire $70–150; replace relay with correct specification $30–80; correct aftermarket wiring $50–200; replace pump if motor is shorted $150–300. Total typical $70–400.
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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.