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A thermostat of the wrong opening temperature fitted. The engine's operating temperature is a calibration target, not a preference — the module's fuelling, timing and emissions strategies all assume the coolant reaches and holds a specific value.

A cooler thermostat is the common error, often fitted deliberately in the belief it protects the engine. It does the opposite in normal use: the engine runs below its target, the module reports that operating temperature was not reached, warm-up enrichment persists, economy falls, and emissions monitors never complete. A vehicle that never quite reaches temperature with a healthy cooling system is this, and the gauge often shows nothing because it is damped.

A hotter one is rarer and less obvious, leaving less margin before overheating. Check the temperature stamped on the thermostat against the specification rather than assuming the fitted part is correct, particularly on a vehicle with recent cooling work — and read actual coolant temperature in live data, where a target that is never reached is unmistakable.