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19 After drilling a pilot hole and a corresponding large hole, a bung for the oxygen sensor is welded to the Y-pipe. The homemade welding job ain’t pretty, but it’s all that’s needed to mount the sensor and connect it to the wiring harness. Ideally, the sensor should be mounted at the top of the exhaust, ahead of the catalytic converter. This isn’t a problem for the late C3s, which have adequate clearance. Wiring for the sensor is routed to the controller.