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Oil pressure that really is below specification — as opposed to a sensor, a switch or a wiring fault reporting that it is. The distinction is the whole point of the name, because the great majority of low-pressure warnings turn out to be the measurement rather than the pressure, and acting on the wrong one of those two is expensive in opposite directions.

Confirm it with a mechanical gauge before anything else. A screw-in gauge at the sender port is the only reading that does not depend on the electronics being right, and it is the difference between a sensor and an engine.

If it is real, the causes run from trivial to terminal: oil level below the pickup, oil thinned by fuel dilution or by being the wrong grade, a blocked pickup screen, a stuck pressure relief valve, a worn pump, or bearing clearances that have opened up with age. Pressure that falls only when hot and at idle is the classic pattern for wear, because hot oil is thinner and idle is the lowest pump speed.

On an engine with cylinder deactivation or variable valve timing, low pressure disables those systems before it damages anything — they are the first consumers to be starved, which is why their codes can be the earliest honest warning that the pressure itself is failing.