P0143 — Oxygen sensor circuit low voltage — bank 1, sensor 3

P0143 indicates signal voltage below the monitor limit for bank 1, sensor 3. This is the third sensor on this bank; its exact position depends on the multi-sensor exhaust layout. This alone does not prove a lean mixture: wiring or air entering the exhaust can alter the signal.

Affected system: Powertrain

Symptoms

• Engine warning lamp and a stored code.
• Driveability symptoms are often absent; emissions inspection issues may occur.

Possible causes

• Signal shorted to ground; on some systems, an open circuit.
• Contaminated sensor, an exhaust leak ahead of it or an actual lean mixture.

What to check

• Confirm B1S3 in the engine diagram and save related codes and freeze-frame data before clearing anything.
• With the exhaust cold, inspect the B1S3 harness for melted insulation, rubbing, water in the connector and loose terminals.
• Check exhaust sealing ahead of the sensor and its signal return; determine whether the reading stays low after warm-up.
• Use the OEM procedure to distinguish a short, a sensor fault and an actual lean condition; avoid judging the part from one reading.
• After correcting the confirmed fault, repeat the OEM monitor conditions and check for code recurrence, beyond merely extinguishing the lamp.

Also written as: P0143-00, P014300, PO143 (some scan tools add the fault-type byte, e.g. -00).