Cummins/Onan 4BT3.9-G2 Engine Engine

This engine also powers 1 machine(s) on Power Equipment Info.

• SWITCH OFF lamp flashes.
  Fix: Indicates Run/Stop/Remote switch is in the Stop position which will prevent automatic starting if an automatic transfer switch is used. Move the Run/Stop/Remote switch to the Remote position for automatic starting.
• OVERCRANK lamp lights and engine stops cranking.
  Cause: Indicates possible fuel system problem.
  Fix: A. Check for empty fuel tank, fuel leaks, or plugged fuel lines and correct as required. b. Check for dirty fuel filter and replace if necessary (see Maintenance section). c. Check for dirty or plugged air filter and replace if necessary (see Maintenance section). d. Reset the control and restart after correcting the problem. Contact an authorized service center for service if none of the above.
• LO FUEL LAMP lights. Engine shuts down and LO OIL PRES lamp lights.
  Fix: Indicates engine has run out of fuel. Check fuel level and replenish as required.
• Engine runs and then shuts down, OVERSPEED lamp lights.
  Cause: Indicates engine has exceeded normal operating speed.
  Fix: Contact an authorized service center for service.
• HI ENG TEMP lamp lights. Engine shuts down.
  Fix: Allow engine to cool down completely before proceeding with the following checks: a. Check coolant level and replenish if low. Look for possible coolant leakage points and repair if necessary. b. Check for obstructions to cooling airflow and correct as necessary. c. Check for a slipping fan belt and tighten if loose. d. Reset control and restart after locating and correcting problem. Contact an authorized service center for service if none of the above.
• The FAULT 1 or FAULT 2 fault lamp lights. Engine shuts down immediately, engine runs for several seconds and then shuts down, or engine continues to run.
  Cause: The standard undesignated fault functions are programmed to shut down the set when a fault is sensed. Fault 1 is time delayed while Fault 2 is immediate. The nature of the fault is an optional selection that is determined when the set installation is designed. The undesignated fault functions may also be programmed for non-shutdown or non-time delay.
• LO FUEL lamp lights. Engine continues to run.
  Cause: Indicates diesel fuel supply is running low.
  Fix: Check fuel supply and replenish as required.
• The external alarm and fault lamp may be deactivated by moving the Run/Stop/Remote switch to the Stop position and pressing the Reset/Lamp Test/Preheat switch.
  Fix: Locate the problem and correct it before restarting the set. While pressing the Reset/Lamp Test/Preheat switch, make certain that all lamps light.
• PRE LO OIL PRES lamp lights. Engine continues to operate.
  Cause: Indicates engine has overheated (engine temperature has risen above 222F/106C) or coolant level is low (sets with coolant level sensor).
  Fix: A. Check coolant level and replenish if low. Look for possible coolant leakage points and repair if necessary. b. Check for obstructions to cooling airflow and correct as necessary. c. Check for a slipping fan belt and tighten if loose. d. Reset control and restart after locating and correcting problem. Contact an authorized service center for service if none of the above.
• PRE HI ENGINE TEMP lamp lights. Engine continues to operate.
  Cause: Indicates engine has begun to overheat and engine temperature has risen to approximately 215F (102C).
  Fix: A. Reduce load if possible by turning off non-critical loads. b. Check air inlets and outlets and remove any obstructions to airflow. If engine can be stopped, follow procedure in step 3.
• LO OIL PRES lamp lights. Engine shuts down.
  Cause: Indicates engine oil pressure has dropped to 20 psi (138 kPa).
  Fix: If generator is powering critical loads and cannot be shut down, wait until next shutdown period and then follow step 5 procedure. If engine can be stopped, follow procedures in step 5.
• LO OIL PRES lamp lights.
  Cause: Indicates engine oil pressure has dropped to 14 psi (97 kPa).
  Fix: Check oil level, lines and filters. If oil system is OK but oil level is low, replenish. Reset control and restart. Contact an authorized service center if oil pressure is not in the range of 25 to 40 psi (172 to 275 kPa).
• LO ENG TEMP lamp lights. Set is in standby mode but is not operating.
  Cause: Lamp lights when engine coolant temperature is 70 F (21 C) or lower. Since the lamp goes out after the engine warms up, there should be no cause for alarm even during initial generator set operation.
• Engine fails to start or starts, runs and then stops
  Fix: Refer to Troubleshooting charts in the Operator’s Manual
• Fault lamp lights but no fault exists. Engine gauges show oil pressure, engine temperature, and frequency (speed) are within normal limits.
  Cause: The monitor board or a sensor may be at fault.
  Fix: Contact your distributor for service.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan 4BT3.9-G2.

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