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The fuel system pressure is below the minimum required for proper injection. Low fuel pressure prevents adequate fuel delivery to the injectors, causing hard starting, loss of power, rough running, and potential engine stalling under load. Modern common-rail systems require precise fuel pressure for proper atomization.
SPN-94 reports engine fuel delivery pressure (low-pressure side). The FMI (Failure Mode Identifier) shown beside it says how the reading is wrong. FMI meanings follow the SAE J1939 standard used by Cummins and most modern diesel engine controllers.
SPN-94 FMI 1
Fuel delivery pressure far below normal (most severe).
Check first: Fuel level, blocked filters, a failing lift pump, air leaks on the suction side.
SPN-94 FMI 18
Fuel delivery pressure below normal (moderately severe).
Check first: Fuel filters and water separator, lift pump output, fuel line restrictions.
SPN-94 FMI 2
The low-side fuel pressure reading is unstable — it swings while the engine runs steadily.
Check first: Air entering the suction side (loose filter bowl, cracked line) can cause this as well as a wiring fault; check both.
SPN-94 FMI 3
The fuel delivery pressure sensor is reporting a voltage too high to be real — usually a broken wire.
Check first: The sensor connector at the fuel filter head; diesel spills during filter changes are the usual cause of corrosion here.
SPN-94 FMI 4
The fuel delivery pressure sensor is reporting a voltage too low to be real — usually a short to ground.
Check first: The harness run past the filter housing and lift pump, where it can be crushed when filters are changed.
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The most common cause is a restricted fuel filter. Fuel filters should be replaced at the manufacturer recommended intervals or sooner if fuel quality is poor.
Check for bubbles in clear fuel line sections, listen for sucking sounds at connections, and inspect all fittings on the suction side (tank to transfer pump) for tightness.
Send us the code or alarm and what the generator is doing, and one of our engineers will help you diagnose it.
Search other controller alarms and engine fault codes, with troubleshooting steps, in Generator Oracle.
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A fault code names a symptom, not a cause. Our engineers have written the diagnostic sequence they use on site for this class of fault — ranked causes, ordered steps with the reading expected at each one, and the point at which to stop and call someone.
8 diagnostic steps · intermediate · technician
10 diagnostic steps · intermediate · technician
9 diagnostic steps · intermediate · technician