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A thermostat that opens above its rated temperature, so the engine runs hotter than intended without overheating. It is the subtler half of a thermostat fault — a stuck-closed one overheats obviously and quickly, while one opening late simply shifts the whole operating band upward.

The consequence is knock rather than a temperature warning. Higher combustion temperatures promote detonation, the knock sensors hear it, and the module retards timing — so the complaint is a flat engine with knock codes and a gauge that looks normal. Read actual coolant temperature in live data rather than the gauge, which is damped across a wide band on most vehicles. Compare against the thermostat's rated opening temperature, since a value consistently above it is the finding.