Cummins/Onan QSJ5.9G Engine

How to read your Cummins/Onan engine serial & model number →

• Engine Oil Pressure Low (Shutdown) — Engine oil pressure is below the low oil pressure shutdown threshold (less than 10 psig for more than 2 seconds) 415
  Cause: Lubricating oil level is low
  Fix: Check the oil level; add or drain oil, if necessary
• Engine Coolant Temperature Above Normal (Warning) — Engine coolant temperature has exceeded the warning threshold (220 °F [105 °C]) for high coolant temperature 146
  Cause: High ambient temperature
  Fix: Measure the air temperature entering the air intake louver of the generator set enclosure if enclosed, or cooling air to the radiator if not enclosed; verify the expected cooling system ambient temperature capability; inspect for recirculation of cooling discharge air into the generator set enclosure air inlet or the area surrounding the radiator cooling air inlet
• Mechanical or internal component wear or damage
  Cause: Internal engine damage
  Fix: Analyze the oil and inspect the filters to locate an area of probable damage; inspect and further investigate area of probable damage
• Fuel Consumption Is Excessive
  Cause: Excessive loads
  Fix: Make sure that the load on the generator set does not exceed the generator set kW rating; re-visit the generator set sizing process to make sure that the generator set is correctly sized for the application, especially if new loads have been introduced into the system; refer to the T-030 manual for proper generator set sizing and application
• Fuel Pressure Low — This fault is used when an optional low fuel pressure switch is installed 2118
  Cause: Low fuel pressure
  Fix: Check fuel supply and change as required
• Engine Shuts Off Unexpectedly or Dies during Deceleration
  Cause: Emergency Stop (if used)
  Fix: Make sure that the Emergency Stop circuit is not energized
• Engine Is Difficult to Start or Does Not Start
  Cause: Battery voltage is low, interrupted, or open
  Fix: Check the battery connections, unswitched battery supply circuit, and fuses; correct or replace if necessary
• Engine Experiences Low Power, Poor Acceleration, or Poor Response
  Cause: Excessive load
  Fix: Ensure that the load on the generator set does not exceed the generator set kW rating; revisit the generator set sizing process to ensure that the generator set is correctly sized for the application, especially if new loads have been introduced into the system
• Unstable Voltage (With Load)
  Cause: Engine governing unstable
  Fix: Refer to the engine service manual (if applicable) or E-Controls service manual
• Poor Voltage Regulation (With Load)
  Cause: Unbalanced load
  Fix: Check voltage and load current on all phases; if unbalanced, redistribute the load more evenly across the phases
• Low Fuel (Warning or Shutdown) — This fault is used when an optional low fuel level switch is installed 1318
  Cause: Low fuel pressure
  Fix: Measure the fuel pressure at the fuel pump inlet
• Engine Starts but Will Not Keep Running
  Cause: Battery voltage is low, interrupted, or open
  Fix: Check the battery connections, unswitched battery supply circuit, and fuses; correct or replace if necessary
• Engine Runs Rough or Misfires
  Cause: Ignition system issues
  Fix: Inspect the spark plugs, clean or replace the spark plugs if necessary; test the ignition coils using a coil on a plug tester (or in-line spark tester) to isolate the faulty coil, replace the faulty coil if necessary; check the coil wiring, inspect connectors and wires for breaks, and test continuity
• Unstable Voltage (No Load)
  Cause: Engine governing unstable
  Fix: Check for engine governor hunting with a frequency meter or tachometer, or cyclic irregularities in the engine with InPower; if any engine DTC fault codes are present, troubleshoot them using the E-Controls manual; check fuel supply pressure to ensure that it is consistent; ensure that the throttle assembly moves freely, clean it if necessary; check the spark plugs for buildup, ensure that a spark is present at each cylinder, repair or replace them if necessary; check for any exhaust restriction, clean or replace components as necessary; check for an intake leak, repair or replace components as necessary; check for proper function of the crankshaft and camshaft position sensors, and the wiring to the ECM from the sensors, repair or replace components as necessary; check the fuel mix control valve for proper operation, such that it can move freely and easily, clean or replace the valve if necessary
• Remote E-Stop — The state of the Remote Emergency Stop has been changed 1434
  Cause: The Remote Emergency Stop button has been activated
  Fix: Reset the Remote Emergency Stop

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan QSJ5.9G.

Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Parts cataloged across all variants of this engine family — verify the exact part fits your specific variant and serial number before ordering.