Cause
Cracked or leaking coolant expansion tank
The expansion tank cracked or leaking at a seam or fitting. The tank is a pressure vessel, not a reservoir — it holds the system's pressure and lets coolant expand — so a crack means the system cannot hold pressure at all.
Losing pressure lowers the boiling point, so the engine can boil at a temperature it would otherwise tolerate, and the loss is gradual enough to be missed. These are moulded plastic and embrittle with heat cycling, cracking at seams and around the neck. A hairline crack often only opens when hot and pressurised, so inspection cold finds nothing — pressure-test instead. Check the cap at the same time, since a cap that no longer holds its rated pressure produces identical symptoms and is a fraction of the cost.
Typically an easy fix. Often worth checking before paying anyone to look.
Some of these codes mean you should stop driving
1 of the 1 codes this can cause indicate a fault where continuing to drive risks damage or is unsafe. Which one you have matters more than the cause does — read the code's own page before deciding.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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