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SPN 157 FMI 4

Injector Metering Rail 1 Pressure (Fuel Rail) — Voltage below normal or shorted low.
The sensor circuit voltage is lower than it should ever be, usually near zero.

What the SPN covers: High-pressure fuel rail pressure on common-rail engines. More on SPN 157.

What FMI 4 usually points to: Signal wire shorted to ground, a lost 5 V reference, or an open signal wire on a pulled-down circuit. More on FMI 4.

Severity

HIGH — limit use, diagnose now

Rail pressure problems can cause stalling, no-start or severe derates. Avoid heavy work until fixed.

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. Low fuel supply pressure (filters, lift pump, air)
  3. Worn high-pressure pump
  4. Leaking injector return or pressure relief valve
  5. Failed rail pressure sensor or wiring
  6. Fuel pressure regulator / inlet metering valve fault

Checks & fixes (easiest first)

  1. Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins
  2. Check the reference voltage, ground and signal at the component per OEM pin-out
  3. Start with fuel supply: filters, priming, air in fuel
  4. Compare desired vs. actual rail pressure on a scan tool
  5. Perform injector return flow test per OEM
  6. Inspect the rail pressure sensor connector

FAQ

What does SPN 157 FMI 4 mean?

Injector Metering Rail 1 Pressure (Fuel Rail) — voltage below normal or shorted low. The sensor circuit voltage is lower than it should ever be, usually near zero.

Is SPN 157 FMI 4 serious?

Rail pressure problems can cause stalling, no-start or severe derates. Avoid heavy work until fixed.

What should I check first for SPN 157 FMI 4?

Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins; Check the reference voltage, ground and signal at the component per OEM pin-out; Start with fuel supply: filters, priming, air in fuel.