Cause
Thermostat stuck closed
A thermostat jammed shut, so coolant cannot circulate to the radiator. It is the failure mode that overheats an engine quickly, and it does so with a full cooling system, a working pump and clear radiator — which is why the usual checks all pass.
The signature is specific and worth knowing: the top hose gets hot while the bottom hose stays cold. Coolant is circulating within the engine and not reaching the radiator, so the difference across the radiator is the whole diagnosis and takes seconds to feel. Temperature climbs steadily regardless of speed, unlike an airflow problem which eases at speed.
It is genuinely urgent. An engine run with a closed thermostat reaches damaging temperatures fast, and the sequence ends in a head gasket or worse. Do not diagnose it by continuing to drive. Overheating also produces knock codes and sensor faults as consequences, so a cluster of unrelated codes on an engine that has run hot should be read as one event.
Typically a moderate job.
Some of these codes mean you should stop driving
1 of the 2 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 (2)
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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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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