Cummins/Onan 6CT 8.3 troubleshooting: 8 fault codes and 179 symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan 6CT 8.3 — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Code | Meaning |
|---|---|
| 31 | Engine will not shut off |
| 38 | Excessive engine moisture |
| 86 | Excessive fuel consumption |
| 387 | Excessive vibration |
| 80 | Exhaust smoke excessive under load |
| 338 | Knock |
| 29 | Fuel or oil leaking from exhaust manifold |
| 35 | Low power |
| Symptom | Cause | Remedy |
|---|---|---|
| Engine Will Not Crank or Cranks Slowly | Starting Motor Operating But Not Cranking the Engine | Remove the starting motor and check for broken teeth on the flywheel or broken spring. |
| Engine Will Not Crank or Cranks Slowly | Engine Drive Units Engaged | Disengage driven units and check for loading from malfunctioning accessories. |
| Engine Will Not Crank or Cranks Slowly | Crankshaft Rotation Restricted | Bar the engine to check for rotational resistance. |
| Engine Will Not Crank or Cranks Slowly | Starting Circuit Connections Loose or Corroded | Check and tighten connections. |
| Engine Will Not Crank or Cranks Slowly | Battery Charge Low | Check battery voltage. |
| Engine Will Not Crank or Cranks Slowly | No Voltage to Starter Solenoid | Check voltage to solenoid. |
| Engine Will Not Crank or Cranks Slowly | Solenoid or Starting Motor Malfunction | Replace starting motor. |
| Engine Hard to Start or Will Not Start (Exhaust Smoke Present) | Engine Cranking Speed to Slow | Refer to Engine Will Not Crank or Cranks Slowly |
| Engine Hard to Start or Will Not Start (Exhaust Smoke Present) | Injection Pump Timing Incorrect | Verify timing/time the injection pump. |
| Engine Hard to Start or Will Not Start (Exhaust Smoke Present) | Valves Incorrectly Adjusted | Adjust valves. |
| Engine Hard to Start or Will Not Start (Exhaust Smoke Present) | Cold Weather or Not Working Properly | Check-repair or replace cold starting aid, if necessary. |
| Engine Hard to Start or Will Not Start (Exhaust Smoke Present) | Air in the Fuel System | Bleed the fuel system and check for suction leaks. |
| Engine Hard to Start or Will Not Start (Exhaust Smoke Present) | One or More Injectors Worn or Malfunctioning | Check/replace injectors. |
| Engine Hard to Start or Will Not Start (Exhaust Smoke Present) | Fuel Supply Restricted | Clean or replace pre-filters and screens and check fuel lines for restrictions. |
| Engine Hard to Start or Will Not Start (Exhaust Smoke Present) | Intake Air System Restricted | Check intake air system for restrictions. |
| Engine Hard to Start or Will Not Start (Exhaust Smoke Present) | Fuel Contaminated | Verify by operating engine from a temporary supply tank. |
| Engine Cranks, But Will Not Start - No Smoke From Exhaust | No Fuel in Tank | Add fuel. |
| Engine Cranks, But Will Not Start - No Smoke From Exhaust | Fuel Shutoff Valve Open | Check for loose wires and verify that the solenoid is functioning. Check to be sure manual shut off lever is in the run position. |
| Engine Cranks, But Will Not Start - No Smoke From Exhaust | If the Condition Occurs During Initial Start, Following an Extended Period of Non Use, or After Replacement of a Fuel System Component, Air in the Fuel System | Bleed the fuel system. |
| Engine Cranks, But Will Not Start - No Smoke From Exhaust | If Previous Engine Operation Had Been Normal, Injection Pump Not Getting Fuel | Loosen the bleed plug at the filter head end. Operate the hand primer on the lift pump to check for fuel. Clean-replace lift pump if necessary. |
| Engine Cranks, But Will Not Start - No Smoke From Exhaust | Fuel Filter Plugged With Water or Other Contamination | Drain Fuel-Water Separator or Replace Fuel Filter. |
| Engine Cranks, But Will Not Start - No Smoke From Exhaust | Worn or Malfunctioning Injection Pump | Loosen the high line at the injectors and previously check hole every while cranking the engine. Replace the pump if fuel is not being delivered. |
| Engine Cranks, But Will Not Start - No Smoke From Exhaust | Injection Pump Timing Incorrect | Verify timing/time the injection pump. |
| Engine Starts But Will Not Keep Running | Idle Speed Too Low | Adjust idle speed. |
| Engine Starts But Will Not Keep Running | Engine Starting Under Load | Disengage driven units and check for loading from malfunctioning accessories. |
| Engine Starts But Will Not Keep Running | Fuel Contaminated | Verify by operating engine from a temporary supply tank. |
| Engine Starts But Will Not Keep Running | Injection Pump Out of Time | Check/correct pump timing. |
| Engine Starts But Will Not Keep Running | Malfunctioning Engine Shut Down Device | Make sure shutdown device is not occurring too soon. |
| Engine Starts But Will Not Keep Running | Air in the Fuel System | Bleed the fuel system and check for suction leaks. |
| Engine Starts But Will Not Keep Running | Fuel Filter Plugged or Fuel Waxing Due to Cold Weather | Drain fuel-water separator or replace filter. Check for fuel waxing in cold weather. |
| Engine Starts But Will Not Keep Running | Fuel Supply Restricted | Clean or replace pre-filters and screens and check for fuel line restrictions. |
| Engine Will Not Shut Off | Electrical or Manual Fuel Shutoff Not Closing | Verify the solenoid is not being energized by a short in the wiring. Check the voltage to the shutoff lever for grounding. Check for the absence of the spring in the pump. Be sure the lever is in the shutoff position. |
| Engine Will Not Shut Off | Engine Running on Fumes Drawn into Air Intake | Locate and isolate the source of fumes. |
| Engine Will Not Shut Off | Fuel Leaking to Intake Manifold | Check for porosity between the fuel filter mounting to the intake manifold. |
| Rough Idle, Warm Engine | Idle Speed Set too Low | Check/adjust low idle screw setting. |
| Rough Idle, Warm Engine | Air in Fuel System | Bleed fuel system and check for suction leaks. |
| Rough Idle, Warm Engine | Injection Timing Incorrect | Check/correct injection timing. |
| Rough Idle, Warm Engine | Injector Nozzles Plugged or Inoperative | Replace injectors. |
| Rough Idle, Warm Engine | Broken Engine Mounts | Replace mounts. |
| Rough Idle, Warm Engine | Wrong High Pressure Lines | Install correct lines. |
| Rough Idle, Warm Engine | Injection Pump Worn or Malfunctioning | Replace pump. |
| Engine Surges at Idle | Low Fuel Level in the Tank | Fill supply tank. |
| Engine Surges at Idle | Idle Speed Set too Low | Check/adjust low idle screw. |
| Engine Surges at Idle | Air in the Fuel System | Bleed fuel system and check for suction leaks. |
| Engine Surges at Idle | Restricted Fuel Supply | Clean or replace pre-filters and screens and check fuel lines for restriction. |
| Engine Surges at Idle | Malfunctioning or Worn Injection Pump | Remove pump. Have pump recalibrated or replace pump. |
| Lubricating Oil Pressure Low | Incorrect Oil Level | Add or drain engine oil. |
| Lubricating Oil Pressure Low | Oil Viscosity Low | Refer to coolant temperature above normal chart. |
| Lubricating Oil Pressure Low | Oil Pressure Gauge Malfunction | Check oil pressure gauge. |
| Lubricating Oil Pressure Low | Regulating Valve Stuck Open or Broken Spring | Check and clean. Replace spring if broken. |
| Lubricating Oil Pressure Low | Oil Diluted with Coolant (Anti-Freeze) | Check for external leak at rear of cylinder head, along fuel pump side of block, oil cooler cover and gear housing. Remove and check water pump plunger seal. Replace leaking pump. Change oil. |
| Lubricating Oil Pressure Low | Oil Filter Plugged | Change oil and replace filter. |
| Lubricating Oil Pressure Low | Incorrect Oil Specifications | Check oil specifications. |
| Lubricating Oil Pressure Too High | Malfunctioning Pressure Switch/Gage | Verify the Pressure Switch is Functioning Correctly. |
| Lubricating Oil Pressure Too High | Engine Running Too Cold | Refer to Troubleshooting Logic for Coolant Temperature Below Normal |
| Lubricating Oil Pressure Too High | Oil Viscosity Too Thick | Make Sure the Correct Oil is Being Used. |
| Lubricating Oil Pressure Too High | Relief Valve Stuck Closed | Check/Replace Valve. |
| Lube Oil Loss | External Leaks | Visually Inspect for Oil Leaks. |
| Lube Oil Loss | Crankcase Being Overfilled | Verify that the Dipstick is Correctly Marked. |
| Lube Oil Loss | Incorrect Oil Specification or Viscosity | Make Sure the Correct Oil is Being Used. |
| Lube Oil Loss | Viscosity from Dilution with Fuel | Look for Reduced Viscosity from Dilution with Fuel. Review/Reduce the Oil Change Intervals. |
| Lube Oil Loss | Oil Cooler Leak | Check for Oil in the Coolant. |
| Lube Oil Loss | If Equipped, Air Compressor Pumping Oil | Check/Replace Air Compressor. |
| Lube Oil Loss | High Blowby Forcing Oil Out the Breather | Check the Breather Tube Area for Signs of Oil Loss. Measure the Blowby and Perform the Required Repairs. |
| Lube Oil Loss | Turbocharger Leaking Oil | Inspect the Turbocharger, the Oil Outlet for Evidence of Oil Transfer. |
| Coolant Temperature Above Normal | Low Coolant Level | Add coolant. |
| Coolant Temperature Above Normal | Radiator Fins Damaged or Obstructed with Debris | Inspect radiator fins - clean or repair if necessary. |
| Coolant Temperature Above Normal | Collapsed Radiator Hoses | Inspect hoses - replace if necessary. |
| Coolant Temperature Above Normal | Cold Weather Radiator Cover Closed | Open radiator cover. |
| Coolant Temperature Above Normal | Incorrect Oil Level | Add or drain engine oil. |
| Coolant Temperature Above Normal | Cooling Fan Shroud Damaged or Missing | Inspect shroud; repair-replace or install. |
| Coolant Temperature Above Normal | Loose Fan Drive Belt | Check the belt tensioner. |
| Coolant Temperature Above Normal | Fan Not Engaging | Check fan sensor. |
| Coolant Temperature Above Normal | Incorrect or Malfunctioning Radiator Cap | Check the radiator cap. Replace if necessary. |
| Coolant Temperature Above Normal | Shutters Are Not Opening Completely or Cold Weather Radiator Cover Closed | Inspect the shutters - repair or replace if necessary. Open radiator cover. |
| Coolant Temperature Above Normal | Temperature Gauge Malfunction | Test the gauge-repair or replace if necessary. |
| Coolant Temperature Above Normal | Incorrect Thermostat | Check/replace thermostat. |
| Coolant Temperature Above Normal | Overfueled Injection Pump | Check/replace pump. |
| Coolant Temperature Above Normal | Vehicle Overloaded | Reduce to a lower gear. |
| Coolant Temperature Above Normal | Air in Cooling System | Measure cylinder block coolant pressure. Check/replace water pump. Check hose clamps for leaks on the suction side of the pump. Check for compression leak through cylinder head gasket. |
| Coolant Temperature Above Normal | Cooling Passages Restricted or Cylinder Head Gasket or Cylinder Block | Flush the cooling system, fill with new coolant. |
| Coolant Loss | Radiator or Cab Heater Leaking | Visually inspect the Radiator Heater, Hoses and Connections to Locate the Leak. If Oil is Present in the Coolant, Check for a Transmission or Lube Oil Cooler Leak. |
| Coolant Loss | External Engine Leak | Visually Inspect the Engine and Components for Seal or Gasket Leaks. |
| Coolant Loss | Overheating or Leaking Compression Gases Resulting in Loss Through the Radiator Overflow | Review the Operation for Overheating and Low Power. Refer to Troubleshooting Logic for Coolant Temperature Above Normal. |
| Coolant Loss | If Equipped, Transmission Cooler Leak | Check for Mixing of Coolant and Transmission Fluid. |
| Coolant Loss | If Equipped with a Water Cooled Air Compressor, Leaking Head or Head Gasket | Look for Coolant in the Oil. Check/Replace Head or Gasket |
| Coolant Loss | Lube Oil Cooler Leak | Check/Replace the Oil Cooler. Look for Coolant in the Oil. |
| Coolant Temperature Below Normal | Radiator Shutters Stuck in Open Position | Inspect the shutters - repair or replace if necessary. |
| Coolant Temperature Below Normal | Fan Malfunction | Check fan for continuous operation. |
| Coolant Temperature Below Normal | Temperature Sensor Malfunction | Check/clean sensor and coolant passage. Test/replace sensor. |
| Coolant Temperature Below Normal | Temperature Gauge Malfunction | Test the gauge - repair or replace if necessary. |
| Coolant Temperature Below Normal | Thermostats Incorrect or Malfunctioning | Check thermostats and replace if necessary. |
| Contaminated Coolant | Without Correct Mixture of Antifreeze and Water | Drain and Flush the Cooling System. Fill with Correct Mixture of Antifreeze and Water. Review the Coolant Change Interval. |
| Contaminated Coolant | If Equipped, Leaking Transmission Oil Cooler | Check/Replace Oil Cooler. Refer to Equipment Manufacturer's Procedures. |
| Contaminated Coolant | Oil Leaks from Oil Cooler, Head Gasket, Head and Cylinder Block | Refer to Troubleshooting Logic for Lube Oil Loss |
| Contaminated Lube Oil | Coolant in the Oil, Internal Engine Component Leaks | Refer to Troubleshooting Logic for Coolant Loss |
| Contaminated Lube Oil | Excessive Oil Sludge | Review Oil and Filter Change Intervals. Make Sure the Correct Oil is Being Used. |
| Contaminated Lube Oil | Worn Injection Pump Plungers | Replace the Pump. |
| Contaminated Lube Oil | Fuel in the Oil, Engine Operating Too Cold | Review the Operation for excessive Idling Resulting in the Engine Running Below Normal Temperature. |
| Contaminated Lube Oil | Lift Pump Seal Leaking | Replace Lift Pump. |
| Contaminated Lube Oil | Injector Needle Valves | Locate and Replace Malfunctioning Injector. |
| Fuel or Oil Leaking from Exhaust Manifold | Intake Air Restriction | Check/Replace Filter Element. |
| Fuel or Oil Leaking from Exhaust Manifold | Excessive Idling | Review Operation for Excessive Idling. |
| Fuel or Oil Leaking from Exhaust Manifold | Injector Needle Valve Stuck Open | Locate and Replace Malfunctioning Injector. |
| Fuel or Oil Leaking from Exhaust Manifold | Obstructed Turbocharger Drain Line | Check/Clean Line. |
| Fuel or Oil Leaking from Exhaust Manifold | Turbocharger Seals Leaking Oil | Check/Replace Turbocharger. |
| Exhaust Smoke Excessive Under Load | Lugging Engine | Use lower gear. |
| Exhaust Smoke Excessive Under Load | More Than One Seal Washer Under Injector | Remove extra washers. |
| Exhaust Smoke Excessive Under Load | Air in Fuel System | Bleed fuel system and check for suction leaks. |
| Exhaust Smoke Excessive Under Load | Injector Nozzle Malfunctioning | Remove and have injector nozzle tested. Replace injectors if necessary. |
| Exhaust Smoke Excessive Under Load | Air Cleaner Plugged | Inspect air cleaner. Clean and replace. |
| Exhaust Smoke Excessive Under Load | Air Fuel Control Line Leak | Check/tighten fittings. |
| Exhaust Smoke Excessive Under Load | Injection Pump Timing Incorrect | Check and adjust injection pump timing. |
| Exhaust Smoke Excessive Under Load | Air leak Between Turbocharger and Intake or Exhaust Manifold | Correct leak. |
| Exhaust Smoke Excessive Under Load | Malfunctioning Turbocharger | Replace turbocharger. |
| Exhaust Smoke Excessive Under Load | Engine Running Too Cold | Check thermostat and cooling system. |
| Engine Will Not Reach Rated Speed When Loaded | Vehicle Overloaded | Reduce load or use lower gear. |
| Engine Will Not Reach Rated Speed When Loaded | Throttle Linkage Adjustment Wrong | Check throttle linkage adjustment for full travel to the high idle stop screw. |
| Engine Will Not Reach Rated Speed When Loaded | Tachometer Has a Malfunction | Check with hand or digital tachometer. |
| Engine Will Not Reach Rated Speed When Loaded | Air Fuel Control Tube Leaking and/or Restricted | Tighten fittings. Replace tube if necessary. |
| Engine Will Not Reach Rated Speed When Loaded | Fuel Supply Restricted | Change the fuel filter and check fuel line, pre-filter and screens for restrictions. |
| Engine Will Not Reach Rated Speed When Loaded | High Speed Screw Incorrectly Adjusted | Check/adjust high speed screw. |
| Engine Will Not Reach Rated Speed When Loaded | Partially Engaged Shutdown Lever | Check/adjust shutdown lever. |
| Engine Will Not Reach Rated Speed When Loaded | Malfunctioning Injection Pump | Replace injection pump. |
| Low Power | Vehicle Overloaded | Reduce load or use lower gears. |
| Low Power | Lubricating Oil Level Too High | Drain oil to proper level. |
| Low Power | Throttle Linkage Adjustment Wrong | Check and adjust throttle linkage for full travel of the fuel control lever. |
| Low Power | Intake or Exhaust System Restricted | Check intake and exhaust systems for restrictions. Inspect air filter and replace as necessary. |
| Low Power | Turbocharger and Intake Manifold | Check intake and exhaust systems for restrictions. Inspect air filter and replace as necessary. |
| Low Power | Air Fuel Control Tube Leaking and/or Restricted | Tighten fittings. Replace tube if necessary. |
| Low Power | Air Leak Between Turbocharger and Intake Manifold | Correct leak. |
| Low Power | Exhaust Leak Between Turbocharger and Exhaust Manifold | Correct leak. |
| Low Power | Air in the Fuel System | Bleed the fuel system and check for suction leaks. |
| Low Power | Fuel Quality Poor | Verify by operating engine from a temporary tank that contains good fuel and refer to fuel oil specifications. |
| Low Power | Fuel Supply Restricted | Clean pre-filters and screens and check fuel line for restriction. Replace fuel filter. |
| Low Power | Malfunctioning Lift Pump | Check/replace lift pump. |
| Low Power | High Intake Air Temperature - (Above 36 Deg.C [96 Deg.F]) | Use outside air to turbocharger in warm weather. |
| Low Power | Low Intake Air Temperature - (Below 0 Deg.C [32 Deg.F]) | Use intake air from under hood in cold weather. |
| Low Power | High Fuel Temperatures (Above 70 Deg.C [168 Deg.F]) | Fill tanks, turn off fuel heater. |
| Low Power | Injection Pump Timing Not Correct | Check timing. |
| Low Power | Turbocharger Worn or Malfunctioning | Replace turbocharger. |
| Low Power | Valves Not Set Properly | Adjust valves. Inspect push rods, springs, etc. |
| Low Power | Injector Nozzles Worn or Malfunctioning | Replace injectors. |
| Low Power | Worn or Malfunctioning Injection Pump | Replace injection pump. |
| Engine Misfiring | Contaminated Fuel | Verify by operating from a temporary tank. |
| Engine Misfiring | Air in Fuel System | Bleed fuel system and check for suction leaks. |
| Engine Misfiring | Fuel Injection Lines Leaking | Inspect and replace broken lines. |
| Engine Misfiring | Incorrect Valve Adjustment | Inspect push rods, springs, etc. Adjust valves. |
| Engine Misfiring | Injector Nozzles Plugged or Inoperative | Replace injectors. |
| Engine Misfiring | Injection Pump Timing Incorrect | Check/time injection pump. |
| Fuel Knock | Engine Overloaded | Use lower gear. |
| Fuel Knock | Poor Quality Fuel | Drain tank; fill with correct fuel. |
| Fuel Knock | Air in Fuel System | Bleed fuel system and check for suction leaks. |
| Fuel Knock | Incorrect Injection Pump Timing | Check pump timing. |
| Fuel Knock | Injection Nozzles Malfunctioning | Remove nozzles. Test and repair. |
| Excessive Fuel Consumption | Fuel Leak | Check for source of the leak. |
| Excessive Fuel Consumption | Additional Loading From Malfunctioning Accessories | Check/repair vehicle components, refer to the Equipment Manufacturer's Procedures. |
| Excessive Fuel Consumption | Operator Technique | Review operation for correct gear shifts, proper deceleration, and idling. |
| Excessive Fuel Consumption | Poor Quality Fuel | Make sure quality No. 2 fuel is being used. |
| Excessive Fuel Consumption | Inadequate Intake Air or Exhaust Restriction | Refer to Troubleshooting Logic for Excessive Exhaust Smoke. |
| Excessive Fuel Consumption | Fuel Pump Timing | Check/time the pump. |
| Excessive Fuel Consumption | Worn or Malfunctioning Injectors | Replace injectors. |
| Excessive Fuel Consumption | Valves Not Seating | Check/adjust valves. |
| Excessive Vibration | Engine Not Running Smoothly | Refer to Troubleshooting Logic for rough running or misfiring. |
| Excessive Vibration | Loose or Broken Engine Mounts | Check/replace (Refer to Equipment Manufacturer's Service Instructions) |
| Excessive Vibration | Damaged Fan or Malfunctioning Accessories | Check/replace the vibrating Component (Refer to Equipment Manufacturer's Service Instructions) |
| Excessive Vibration | Malfunctioning Vibration Damper | Inspect/replace the vibration damper. |
| Excessive Vibration | Worn or Damaged Alternator Bearing | Check/replace the alternator. |
| Excessive Engine Noises | Drive Belt Squeal, Insufficient Tension or Abnormally High Loading | Check the tensioner and inspect the drive belt. Make sure water pump, tensioner pulley, fan hub and alternator turn freely. |
| Excessive Engine Noises | Accessory Drive Belt Squeal | Check the tension of accessory drive belts. (Refer to the Equipment Manufacturer's Procedures.) Make sure the accessories turn freely. |
| Excessive Engine Noises | Intake Air or Exhaust Leaks | Refer to Troubleshooting Logic for excessive exhaust smoke. |
| Excessive Engine Noises | Excessive Valve Lash | Adjust valves. Make sure the push rods are not bent or the rocker levers are not severely worn. |
| Excessive Engine Noises | Turbocharger Noise | Check turbocharger impeller and turbine wheel for housing contact. |
| Excessive Engine Noises | Gear Train Noise | Check/replace vibration damper. |
| Alternator Not Charging or Insufficient Charging | Loose or Corroded Battery Connections | Check and clean connections. |
| Alternator Not Charging or Insufficient Charging | Alternator Belt Slipping | Check/replace belt tensioner. |
| Alternator Not Charging or Insufficient Charging | Alternator Pulley Loose on Shaft | Tighten pulley. |
| Alternator Not Charging or Insufficient Charging | Gauge or Lamp Malfunction | Check/Replace the gauge or lamp. |
| Alternator Not Charging or Insufficient Charging | Malfunctioning Alternator | Replace alternator. |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Vehicle Overloaded; Lubricating Oil Level Too High; Throttle Linkage Adjustment Wrong; Intake or Exhaust System Restricted. See the table for the matching fixes.
Documented causes: Low Coolant Level; Radiator Fins Damaged or Obstructed with Debris; Collapsed Radiator Hoses; Cold Weather Radiator Cover Closed. See the table for the matching fixes.
Documented causes: Lugging Engine; More Than One Seal Washer Under Injector; Air in Fuel System; Injector Nozzle Malfunctioning. See the table for the matching fixes.