Engine Coolant Temperature Extremely High
The ECU is detecting that engine coolant temperature has exceeded 115°C / 240°F. This is the most severe level of coolant temperature warning. The ECU will actively derate engine power to protect the engine.
Repair difficulty
DIY — Basic toolsRepair time
15 min
Dealer cost
$200-400
Likely causes
- 1
Radiator or cooler stack blocked with debris— Most likely
DIYThe most common cause, especially during harvest or bush hogging. Clean the entire cooler stack from the engine side outward.
- 2
Low coolant level from leak at hoses, clamps, or water pump weep hole— Most likely
DIYCheck for puddles under the tractor and wet spots around hose connections.
- 3
Failed thermostat stuck closed— Possible
DIYIf the upper radiator hose is cool while the gauge shows hot, the thermostat isn't opening. Replacement takes 30-60 minutes.
- 4
Fan clutch failure (viscous coupling worn out)— Possible
ShopThe fan should spin faster as temperature rises. If it freewheels at all speeds, the clutch has failed.
- 5
Failed water pump (broken impeller or shaft)— Less common
ShopLess common but serious. Shop recommended for replacement.
- 6
Internally blocked radiator— Less common
ShopRequires removal and professional cleaning or replacement. Use an infrared thermometer to check for cold spots indicating blocked passages.
- 7
Head gasket failure— Less common
ShopCheck for milky oil on dipstick, bubbles in coolant reservoir, or sweet smell in exhaust. Major repair.
- 8
Faulty coolant temperature sensor giving false reading— Less common
DIYIf everything else checks out, test the sensor with a multimeter. Resistance should change smoothly with temperature.
Parts needed
Coolant temperature sensor
OEM: RE537637$40
OEM ~$39-51. Aftermarket $11-27. Fits 5E, 5M, 5R, 6R, 7R, 8R, 9R and many more.
Search for this part →Thermostat (180°F / 82°C)
OEM: RE501052$45
Varies by engine model. Confirm before ordering.
Search for this part →Radiator cap (15 PSI)
OEM: AT365870$20
A bad cap lowers the boiling point. Always carry a spare.
Search for this part →Fan clutch assembly
OEM: RE274872$550
Varies by model. Diagnose thoroughly before replacing — significant cost.
Search for this part →Coolant (John Deere Cool-Gard II, 1 gallon concentrate)
OEM: TY26575$30
Mix 50/50 with distilled water. Do NOT use standard automotive antifreeze.
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