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The heat exchanger that keeps transmission fluid within its working temperature, no longer passing enough flow.

Heat is what kills automatic transmissions, and the relationship is steep: fluid life roughly halves for every 10 °C above its design temperature. A cooler that has stopped working does not announce itself — the car drives normally and the damage accumulates invisibly until something slips.

It blocks from both sides, and the outside is free to check. Externally it sits in the airflow at the front of the car collecting leaves, insects and road debris. Internally it clogs with the debris of an earlier transmission problem, which is why a cooler must be flushed or replaced after any failure that put material into the fluid — fitting a rebuilt gearbox onto a cooler full of the old one's debris is a well-known way to destroy both.

A stuck thermostatic bypass is the third case. Many coolers are bypassed until the fluid warms, to avoid the losses of thick cold fluid; a bypass that never closes means the cooler is plumbed in and never used.

It is cheap to inspect and near the top of the list for any over-temperature code, precisely because nothing about the way the car drives will point at it.