Fault codeP2251

MODERATE

O2 Sensor Negative Current Control Circuit/Open (Bank 1 Sensor 1)

What this code means

O2 Sensor Negative Current Control Circuit/Open (Bank 1 Sensor 1).

“Bank 1” is the side of the engine that holds cylinder number 1; “Bank 2” only exists on V and flat engines. An in-line engine has a single bank.

“Sensor 1” is the oxygen sensor upstream of the catalytic converter, the one that regulates the mixture; “Sensor 2” sits downstream and monitors the converter.

P2251 is a generic code: it means the same thing on every make. P2251 belongs to family P22xx — Fuel and air metering sensors. Its page explains how to read a code and groups all its codes by part.

Type of fault

The control unit no longer sees continuity in this control circuit: a broken wire, an unplugged connector, or an open winding in the component. A multimeter checks both the component's resistance and the continuity of the wires.

Oxygen sensor (lambda sensor)

The oxygen sensor is screwed into the exhaust and measures how much oxygen is left after combustion. The upstream sensor (sensor 1) lets the control unit keep the mixture right on the ideal ratio, correcting it many times a second; the downstream sensor (sensor 2), after the catalytic converter, checks that the converter is doing its job.

What you notice

An upstream sensor that fails often raises fuel consumption and can make the idle uneven; a downstream sensor fault is usually felt not at all. In both cases the car may fail the emissions test.

How to check it

Inspect the sensor's connector and wiring, which run close to the hot exhaust and often melt or corrode. An exhaust leak upstream of the sensor lets in outside air and fools it. A sensor coated in soot, oil or white deposits points to a combustion problem to solve first, or the new sensor will die the same way.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

O2 sensor voltage, bank 1 sensor 1
O2 sensor voltage, bank 1 sensor 2
Short term fuel trim bank 1
Long term fuel trim bank 1
Commanded equivalence ratio (lambda)

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

How to read this code

PPowertrain
2standardised code
2Fuel and air metering sensors
51P2251