O2 Sensor Signal Stuck Lean (Bank 1 Sensor 2)

What it means

The engine computer tests the downstream sensor by briefly enriching the mixture and waiting for its voltage to rise. It found that the Bank 1 Sensor 2 signal stayed low (showing lean) anyway. The most common causes are an exhaust leak near the sensor or a weak sensor.

Related codes: P2271P2272P0137P0140P0171

Symptoms4

• Check engine light on, often with no other symptoms.
• A catalytic converter efficiency code may appear with it.
• Failed emissions test.
• Ticking or puffing noise from the exhaust if a leak is the cause.

Possible causes5

• Exhaust leak ahead of or near the downstream sensor letting in outside air.
• Failed or aged downstream oxygen sensor.
• Sensor signal wire shorted to ground or a poor connector.
• A real lean mixture from an intake leak, weak fuel supply or misfire.
• Engine computer fault (rare).

How to diagnose5

1. Fix any lean or misfire codes such as P0171 or P0300 first.
2. Watch the downstream sensor voltage on a scan tool during hard acceleration; it should rise clearly as the mixture goes rich.
3. Check the exhaust for leaks at joints and gaskets ahead of the downstream sensor.
4. Inspect the sensor wiring and connector for a short to ground or contact with the exhaust.
5. If the voltage stays low with a confirmed rich mixture and no exhaust leak, the sensor is faulty.

How to fix5

1. Repair the exhaust leak or replace the damaged gasket.
2. Repair the damaged wire or connector.
3. Fix the cause of any lean condition.
4. Replace the downstream oxygen sensor with a unit that meets the vehicle specification.
5. Clear the codes and drive until the sensor self-test completes.

The cost is low for a small exhaust leak or wiring repair, and reaches the upper end when the downstream oxygen sensor is replaced.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions