C1781: Engine Coolant Temperature Signal Missing/Fault

What does C1781 mean?

C1781 is a diagnostic trouble code indicating "Engine Coolant Temperature Signal Missing/Fault". This code relates to your vehicle's chassis electronics and should be diagnosed to determine the root cause. The severity level helps indicate how urgently this should be addressed.

Symptoms you may notice

• Check engine light (MIL) illuminated on dashboard
• Engine overheating
• Temperature gauge reading high
• Heater not working properly
• Intermittent warning lights

Common causes

• Stuck thermostat (open or closed)
• Low coolant level or coolant leak
• Failed coolant temperature sensor
• Faulty water pump
• Blocked radiator

What should you do?

This code sits in an engine peripheral, typically emissions or forced induction. It will not take braking or steering away, but it can put the car into limp mode, cost you fuel and fail a roadworthy. Book it in rather than clearing it and hoping.