Cummins/Onan 6BT5.9 G1 Engine

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

• The starter will disengage after a specified period of time and the control will indicate an overcrank fault
  Fix: Place the Run-Stop-Remote switch in the STOP position and momentarily depress the Reset switch. Wait two minutes for the Starter motor to cool and then repeat the starting procedure
• The fault lamp lights. Engine shuts down immediately, engine runs for several seconds and then shuts down, or engine continues to run. FAULT 1 or FAULT 2
• SWITCH OFF lamp flashes.
  Fix: Indicates Run-Stop-Remote switch is in the Stop position which will prevent automatic Starting. Moving the Run-Stop-Remote switch to the Remote position will stop the lamp flashing and enable automatic starting.
• PRELO OIL PRES lamp lights. Engine continues to operate.
  Fix: Indicates engine oil pressure has dropped to 20 psi (138 kPa). 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 procedure in step 5.
• PREHI ENGINE TEMP lamp lights. Engine continues to operate.
  Fix: Indicates engine has begun to overheat and engine temperature has risen to approximately 220° F (105° C). If generator is powering non-critical and critical loads, and cannot be shut down, use the following: a. Reduce load if possible by turning off non-Critical loads. b. Check air inlets and outlets and remove any obstructions to airflow. c. Open doors or windows in generator area to increase ventilation. If engine can be stopped, follow procedure in step 3.
• OVERCRANK lamp lights and engine stops cranking.
  Fix: Indicates possible fuel system problem. a. Check for empty fuel tank, fuel leaks, or plugged fuel lines and correct as required. Engine runs, shuts down, and LO OIL PRES lamp lights. 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. Refer to step 5. e. Reset the control and restart after correcting the problem. Contact your distributor for service if none of the above applies.
• No AC output voltage.
  Fix: Voltage regulator is inoperative. Refer to Initial Start and Checks, and Adjustments sections of your Installation Manual for voltage check and adjustment procedures. Contact your distributor if voltage build-up is still a problem.
• LOOIL PRES lamp lights. Engine shuts down.
  Fix: Indicates engine oil pressure has dropped to 14 psi (97 kPa). Check oil level, replenish as necessary. Check oil lines and filters for leaks, repair as required. Reset control and restart. Contact your distributor if oil pressure is not in the range of 35 to 55 psi (241 to 379 kPa).
• LOFUEL lamp lights. Engine continues to run.
  Fix: Indicates diesel fuel supply is running low. Check fuel supply and replenish as required.
• LOENG TEMP lamp lights. Set is in standby mode but is not operating.
  Fix: 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 if it remains lighted during initial generator Set operation. Or is not circulating coolant. Check for the following conditions: a. Coolant heater not connected to power supply. Check for blown fuse or disconnected heater cord and correct as required. b. Check for low coolant level and replenish. if required. Look for possible coolant leakage points and repair as required. c. Contact your distributor if none of the above applies.
• 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.
• HI ENG TEMP lamp lights. Engine shuts down.
  Fix: Indicates engine has overheated (engine temperature has risen above 230° F/110° C) or coolant level is low (sets with coolant level sensor). 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, tighten if loose. d. Reset control and restart after locating and correcting problem. Contact your distributor if none of the above applies.
• Green RUN lamp lights following engine Start-up.
  Fix: Indicates all engine systems are normal. No corrective action required.
• Green RUN lamp does not light following engine start-up.
  Fix: Indicates possible Start/Disconnect relay failure. Contact your distributor for assistance.
• 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 6BT5.9 G1.

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

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