Quick answer: The ECU sees little or no believable signal activity from the downstream oxygen sensor on Bank 1.
Drivers also search this fault as bank 1 sensor 2 no activity, rear O2 no activity bank 1, downstream oxygen sensor dead bank 1.
P0140 means the Bank 1 rear oxygen sensor is not showing the activity the ECU expects. The signal may be flatlined, disconnected, badly biased, or electrically dead enough that the catalyst monitor has effectively lost its downstream witness. Because this is a post-catalyst sensor, the code usually matters more for emissions and catalyst logic than for direct drivability.
P0140 is commonly driveable short-term, but catalyst monitoring and emissions readiness stay unreliable until the Bank 1 rear-sensor activity comes back.
Cause phrases often tied to this code: rear O2 flatline, dead downstream sensor, open circuit, connector unplugged, exhaust wiring issue.
Brand hubs help broaden internal linking now and can evolve into make-specific diagnostic notes later.
Useful when the driver knows the wording but not the exact DTC yet.
Often it means the ECU is seeing no meaningful activity, but the root cause can be the sensor itself, the connector, or the wiring path.
Usually not by itself because it is a downstream sensor code, but it can block readiness and distort catalyst conclusions.
Yes. A dead rear-sensor signal can make catalyst monitoring unreliable, but it does not automatically prove the converter failed.