OBD-II code P2251 indicates an open circuit in the O2 sensor negative current control circuit on Bank 1, Sensor 1 (the upstream sensor ahead of the catalytic converter). This circuit is part of the sensor's internal current-pumping control that the engine control module uses to maintain a precise reference across the sensing cell. When the control module cannot detect the expected current flow in that leg of the circuit, it sets this code.
The upstream O2 sensor is a wear item that degrades from thermal cycling and contamination. Internal failure of the negative current control element is a common end-of-life failure mode and is the single most likely cause.
A break, chafe, stretched conductor, or corroded splice in the specific signal wire between the sensor connector and the engine control module will register as an open circuit to the controller.
Heat, moisture, and vibration at the exhaust-side connector commonly corrode terminals or allow pins to lose tension, producing an intermittent or hard open in the control circuit.
The upstream sensor harness runs near hot exhaust components. Melted insulation, cracked conduit, or a wire pinched against the exhaust manifold can sever the control circuit.
If the sensor's reference ground or heater feed is lost, the negative current control circuit can appear open because the sensor cell cannot complete its pumping action.
An internal open in the module's current-control output stage is rare but possible. It should be considered only after the sensor, wiring, connectors, and grounds have all been verified good.
Often it is, because the sensor is a normal wear item and internal failure of the current control circuit is its most common end-of-life mode. However, you should not replace the sensor until the wiring and connector are inspected, since a corroded connector or broken wire will produce the same code on a brand new sensor.
Online references sometimes list the engine control module as a cause, but in real-world repair data this code is almost never caused by the computer. Module replacement should be considered only as a last resort, after the sensor, the wiring, the connector, and the grounds have all been verified good with a multimeter.
In most cases yes, and drivability is usually normal because the engine can still operate in a default fuel strategy. However, fuel economy and emissions may suffer, and the vehicle may fail an emissions inspection, so it should be repaired within a reasonable time.
Bank 1 Sensor 1 is the upstream sensor, located in the exhaust manifold or downpipe ahead of the catalytic converter on the bank of the engine that contains cylinder 1. The downstream sensor is Bank 1 Sensor 2 and is a different circuit with its own code set.