During the diagnostic process, we occasionally forget that OBD II engine control modules (ECMs) can’t function unless they accurately gather and process data from 5-volt reference sensors. Most engine management sensors are two-wire circuits that contain a 5-volt reference and a signal return wire or three-wire circuits that contain a 5-volt, signal return and auxiliary ground wire.
Author Gary Goms provides two real-world examples of how two- and three-wire sensors can cause an intermittent rich operating condition.
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