OBD-II Fault Code

C1781: Engine Coolant Temperature Signal Missing/Fault

Meaning, symptoms, likely causes, diagnostic checks and repair guidance for C1781.

System family

Chassis

Manufacturer-specific

Repair category

Cooling system

Use this category to narrow your checks before replacing parts.

Severity

High

Severity is a general guide. Confirm with scan-tool data and vehicle symptoms.

What C1781 usually means

Engine Coolant Temperature Signal Missing/Fault.

Common symptoms

ABS/traction/steering/suspension warning lamp or abnormal chassis behaviour; Temperature gauge abnormal; Overheating or slow warm-up possible

Likely causes

Low coolant; Faulty thermostat; Coolant temperature sensor/wiring fault; Cooling fan issue

Diagnostic checks

Inspect wheel/brake/suspension area, connectors, sensor wiring and module power/ground; Check coolant level only when engine is cool; Compare scan-tool temperature to actual temperature

Repair guidance

Repair leaks, replace thermostat/sensor/fan component after confirming fault

Verification after repair

Clear code, road-test safely at low speed first, then rescan

Note: Never open hot cooling system cap; Manufacturer-specific: confirm definition and test steps in the vehicle service manual. A DTC identifies a circuit or system condition, not always the failed part.