O2 Sensor Circuit (Bank 1 Sensor 2)
What it means
The engine computer has found a fault in the signal of the downstream oxygen sensor on bank 1, the sensor that monitors catalytic converter efficiency. This sensor has little effect on fuel control, so the engine usually runs normally, but the computer can no longer monitor the converter accurately.
Related lights
Dashboard lights that may come on with this code.
Symptoms3
- Check engine light on, usually with no other symptom.
- Catalytic converter self-test does not complete.
- Failed emissions test.
Possible causes5
- Wiring or connector damaged by exhaust heat or road impact under the car.
- Failed downstream oxygen sensor.
- Exhaust leak near the sensor.
- Sensor heater fault.
- Engine computer fault (rare).
How to diagnose5
- Read all codes, noting a heater code such as P0141 or the converter efficiency code P0420.
- Watch the downstream signal after warm-up; with a good converter it is calmer than the upstream signal, but it should not be completely flat or out of range.
- Inspect the wiring under the car, which is exposed to heat, stones and water.
- Check the exhaust for a leak near the sensor.
- Check circuit continuity between the sensor and computer using the manufacturer wiring diagram.
How to fix4
- Repair the wiring or connector and secure it with the original clips.
- Repair any exhaust leak.
- Replace the downstream oxygen sensor if it is proven faulty.
- Clear the codes and drive until the sensor and converter tests complete.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
In the Saudi market: SAR 100–900 (≈ USD 27–240)
Our prices come from the Saudi market, converted at a fixed rate (SAR 3.75 per USD). Prices in your country may differ a lot, so we show the cost level.
A wiring repair under the car keeps the cost low; a new downstream sensor raises it, and labor grows if the old sensor is rusted in place.
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
Does this code affect how the car drives?
Rarely, since the downstream sensor monitors the converter and has little control over the mixture, but it must be fixed to pass an emissions test.
Can this fault trigger P0420?
Yes. A wrong downstream signal can make the computer wrongly judge the converter as weak, so fix the sensor before condemning the converter.
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