Any air coming into the system after the MAF/AMM will cause a rich condition at the spark plugs. Initially, the computer sees a lean burn at the front O2 sensor and adjusts the fuel to increase to make up for it. Often associated with a long term lean/rich error, which may or may not trigger a fault.

The rear O2 sensor is just a monitor to make sure the front O2 sensor is doing its job. Some scanners can follow the O2 sensors and graph the electrical flow, but a P0140 is a wiring or plug problem. If the mechanic didn't unplug the sensors before removing them, the wires will get twisted badly and stretched. If the plug end was removed, there may be dirt or grease in the area causing a bad contact.

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