Service Engine Soon Light (MIL) On with Oxygen Sensor Faults Stored. The Service Engine Soon light comes on while driving and, in some cases, engine power output may be reduced. The following fault codes can be stored in the DME memory: ? 12A902 - Oxygen sensor after catalytic convertor, electrical: short circuit to ground ? 12AA02 - Oxygen sensor after catalytic convertor 2, electrical: short circuit to ground ? 12B302 - Oxygen sensor after catalytic convertor, activation: short circuit to ground ? 12B304 - Oxygen sensor after catalytic convertor, activation: line disconnection ? 12AB04 - Oxygen sensor after catalytic convertor, electrical: line disconnection ? 12AC04 - Oxygen sensor after catalytic convertor 2, electrical: line disconnection ? 12A701- Oxygen sensor after catalytic convertor, activation: short circuit to B+ Under certain conditions, moisture can enter the internal insulation layers of the oxygen sensor. If the sensor's heating activation occurs too early, the internal pressure can damage the sensor elements, leading to the faults being set.
