AI4Car quick readA short, original guide from the AI4Car knowledge library.
P0128 is associated with coolant temperature staying below the expected range during the monitor. A thermostat that is not controlling flow correctly is one possibility, but compare live coolant-temperature data with the actual cold-start conditions.
A temperature gauge that behaves strangely, poor cabin heat, or unusually long warm-up are useful details to record. Never open a pressurized cooling system while it is hot.
Quick educational note
For information, not a diagnosis or repair instruction. Vehicle-specific checks are still required. Consult the manufacturer's guidance and a qualified technician.
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This article explains general principles and does not establish what is wrong with your vehicle. Read the content limitations.