SPN 51 FMI 4 — Throttle Position Circuit Voltage Below Normal
TPS low-voltage codes require short isolation and circuit proof. Shorted signal after harness rub-through is the most documented pattern. The FSM enforces short isolation before assembly replacement.
Fault Overview
SPN 51 FMI 4 is a throttle position circuit low voltage or short class fault. Signal short to ground, failed sensor, crushed harness, and water intrusion are the primary causes. Shorted signal after harness rub-through is a documented pattern. Replacing the throttle body assembly before isolating the short source is the most documented misdiagnosis pattern.
Engine & Platform Coverage
Symptoms
- •Throttle position low voltage
- •TPS shorted
- •Intake throttle signal low
- •Actuator not responding from low signal
- •Unstable idle strategies from TPS fault
Possible Causes
- 1.Signal short to ground — signal circuit shorted to ground producing low voltage reading
- 2.Failed sensor — failed sensor producing low voltage reading
- 3.Crushed harness — harness crushed at intake bracketry producing short to ground
- 4.Water intrusion — water intrusion producing short circuit
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
3 steps locked behind Technician Mode- 1Step 1 — Disconnect Isolation: Disconnect the sensor and recheck the circuit. A short that clears with the sensor disconnected confirms a sensor fault. A short that remains with the sensor disconnected confirms a harness fault. Shorted signal after harness rub-through is a documented pattern.
- 2Step 2 — Short Checks: Perform short checks on the harness. Inspect pinch points around intake bracketry. A harness crushed at intake bracketry producing a short to ground requires harness repair.
- 3Step 3 — Reference Voltage Verification: Verify reference voltage remains healthy after disconnect. Signal near 0V with the sensor disconnected confirms a harness short. Verify the reference voltage is not affected by the short.
- 4Step 4 — Replace Sensor or Assembly If Circuit Is Clean: Replace the sensor or assembly if the circuit is clean. Confirm the harness is clean before any replacement. A sensor or assembly replaced into a harness short will fail the new component.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
1 steps locked- 1Repair harness if short is confirmed in the harness.
- 2Service connector if water intrusion is confirmed.
- 3Replace sensor or assembly if failure is confirmed after circuit proof.
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.
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Summary
TPS low-voltage codes require short isolation and circuit proof. The AI Diesel Tech FSM enforces short isolation before assembly replacement.