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Belt-driven accessories loading the engine more than the idle control can absorb. The air conditioning compressor, the power steering pump and the alternator all take real power, and at idle the engine has very little in reserve.

The module anticipates this and raises idle in advance — which is why a healthy vehicle does not stumble when the air conditioning engages. When an accessory takes more than expected, the compensation is insufficient and the idle dips, hunts or stalls, and the module reports that it could not hold its target.

A seizing compressor or a failing pump bearing is the usual cause. Test by isolating: disconnect the compressor clutch and see whether idle recovers, and turn each accessory by hand for stiffness or noise. A dragging alternator or a power steering pump running against a blockage loads the engine the same way — and a tired belt tensioner produces the reverse symptom, letting the belt slip rather than loading the engine.