bypass mode
Bypass filters only act upon a portion of the engine oil flow, typically less than 10%, whereas those that filter the whole stream are known as full-flow filters. In some engine designs, a primary full-flow filter is accompanied by a secondary bypass filter, with the latter filtering particles too small for the primary. This dual-filter design can increase the time between subsequent servicing of the lubrication system.[3]
Many full-flow mechanical filters incorporate an integrated pressure relief valve to allow a bypass mode. If the filtration element becomes completely clogged, this valve allows oil to bypass the filter, protecting the engine from oil starvation. The valve may also open in very cold conditions if a high viscosity oil is used.
Got that passage from oil filters on wikipedia. Bypass mode is also a mode when you talk about pcms. A throttle body bypass is what was eluded to earlier. In what context did you hear bypass mode? Maybe then your question can be answered accurately.
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