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Resistance in a module's power feed or earth, high enough that the module's supply sags under load while measuring perfectly at rest. The distinction from an open circuit is that everything appears to work — the module powers up, communicates and answers questions, and only misbehaves when current is drawn.

A meter's continuity test will not find it, which is the point worth remembering. A joint reading near zero ohms with a few milliamps can drop several volts when the module energises its outputs. Measure voltage drop across the feed and the earth while the module is actually working, comparing the module's supply pin against the battery.

The symptoms are erratic and look like software. A module browning out below its minimum operating voltage resets, loses communication briefly, or reports impossible values from its own outputs — so the codes name actuator supply faults, internal performance, or lost communication, and point everywhere except at a connection. Corroded earth points behind trim or under the vehicle are the commonest cause, and a supplementary earth strap temporarily fitted for a test is a quick way to confirm it.