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A leak in the exhaust close to an oxygen or air-fuel ratio sensor, drawing outside air into the stream the sensor is sampling. Exhaust flow is pulsed, so a leak upstream of a sensor pulls air in during the low-pressure part of each pulse even though the average pressure is positive.

The sensor then reports more oxygen than the engine produced, so the module reads it as lean and adds fuel — and the engine genuinely runs rich as a result. A rich-running engine with a lean-reporting sensor is the signature, and it is the opposite of what people expect.

The leak is usually at a joint rather than a hole: a manifold gasket, a cracked manifold, a flexible section, or the sensor's own boss. Listen for a tick that follows engine speed and is loudest cold, before the metal expands and closes the crack. Check for a leak before replacing any oxygen sensor reporting a persistent lean condition — it is the commonest reason a new sensor changes nothing.