A temperature display that changes abruptly when cabin heating is selected needs clarification: is the actual reported coolant temperature changing, or only the dashboard indication? The distinction influences the next diagnostic step.
Save both the symptom and conditions
Record whether the engine is cold or warm, whether cabin heat remains normal and whether other gauges behave unusually. Include recent cooling-system, dashboard or electrical work. If a garage has compared scan readings with the display, retain its results.
Do not dismiss a possible overheating warning as an electrical fault without assessment. Stop as instructed by the vehicle manual and never open a hot cooling system. A technician should use the correct checks to distinguish genuine thermal behaviour from sensing or display problems. Explain whether the change is gradual or instantaneous and whether it returns when the heater setting changes. That timing is more informative than simply requesting a temperature sensor or ECU.
Related help
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