Ground strap (earth strap)
Definition
Ground straps are the braided or heavy cables that connect the battery negative to the body, the engine to the body, and the transmission or subframe to the body, completing the return path for every circuit on the vehicle. Because the engine sits on rubber mounts, the engine-to-body strap carries the starter's hundreds of amps and the return current of every engine sensor and actuator; when it corrodes or breaks, current finds alternative paths through throttle cables, exhaust hangers, coolant hoses and gearbox cables, which melt or burn. Symptoms of a bad ground are slow cranking, dimming lights, flickering dash, sensor readings that shift with electrical load, lambda and knock sensor codes, erratic gauges and modules dropping off the CAN bus, often worse in wet or cold weather. Test with a voltage drop measurement: with the starter cranking there should be less than 0.2 V between the engine block and the battery negative post, and less than 0.1 V between body and battery negative. Codes caused by poor grounds are indirect and scattered: P0606, P0130/P0135, P0325, U0100/U0121, P0562 and random misfire P0300.