Cause
Blocked or clogged radiator
The main radiator restricted, internally or externally, so it cannot shed heat. Externally it packs with leaves, insects and road debris, invisible from the front because the outer face still looks like a radiator — look through it towards a light.
Internally it silts up with corrosion products and the residue of mixed or exhausted coolant, which blocks individual tubes. The two behave differently and that is the diagnostic split: external blockage overheats in slow traffic and eases at speed, while internal blockage overheats under sustained load regardless of airflow. Feel across the radiator's face once hot — cold patches are blocked tubes, and an even temperature with debris in the fins is the external case.
Typically a moderate job.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
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