O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)

What it means

The engine computer has seen the downstream oxygen sensor voltage on bank 1 stay lower than expected. After the catalytic converter the sensor normally sees gas low in oxygen, so a reading that stays low usually means outside air is reaching it through a leak, a short in its circuit, or a failed sensor.

Related codes: P0136P0138P2270P0420P0141

Symptoms4

• Check engine light on.
• Engine performance is normal in most cases.
• Ticking or puffing noise from the exhaust if there is a leak.
• Failed emissions test.

Possible causes5

• Exhaust leak near or ahead of the downstream sensor drawing in outside air.
• Signal wire shorted to ground under the car.
• Failed downstream oxygen sensor.
• A real lean mixture lasting a long time.
• Engine computer fault (rare).

How to diagnose5

1. Check the exhaust for leaks between the converter and the downstream sensor and at pipe joints.
2. Watch the sensor signal during brief enrichment or hard acceleration; a good sensor voltage rises.
3. Unplug the connector and check the signal wire for a short to ground.
4. Check the fuel trims to rule out a real lean mixture.
5. Inspect the wiring under the car for chafing or melting.

How to fix5

1. Repair the exhaust leak, the most common cause of this code.
2. Repair the wire shorted to ground and secure it in place.
3. Fix any lean mixture cause found.
4. Replace the downstream oxygen sensor if it does not respond to enrichment.
5. Clear the codes and drive until the sensor test completes.

Repair cost

In Saudi riyals; the highlighted part is the expected range for this fault.

An exhaust joint or wiring repair keeps the cost low, while replacing the sensor pushes it to the upper end.

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