O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)
What it means
The engine computer has seen the upstream oxygen sensor voltage on bank 1 stay lower than allowed for too long. A low voltage normally means the sensor sees a lean mixture, so either the mixture really is lean, air is entering ahead of the sensor, or the signal is being pulled to ground by an electrical fault.
Related lights
Dashboard lights that may come on with this code.
Symptoms4
- Check engine light on.
- Higher fuel consumption, since the computer may add extra fuel.
- Fuel smell or dark exhaust smoke in some cases.
- Slightly uneven idle.
Possible causes5
- Signal wire shorted to ground or a damaged connector.
- Failed or contaminated upstream oxygen sensor.
- Exhaust leak ahead of the sensor letting in outside oxygen.
- A real lean mixture from a vacuum leak or low fuel pressure.
- Engine computer fault (rare).
How to diagnose5
- Check the fuel trims; very high trims mean the mixture really is lean, so find that cause first.
- Test sensor response by briefly enriching the mixture under a technician's supervision; a good sensor voltage rises quickly.
- Unplug the connector and check the signal wire for a short to ground or to the vehicle body.
- Inspect the exhaust for leaks ahead of the sensor, especially at the exhaust manifold gasket.
- If the code appears with lean codes on both banks, the cause is usually air or fuel, not the sensor.
How to fix5
- Repair the wiring or connector if a short to ground is found.
- Repair the exhaust leak ahead of the sensor.
- Fix the cause of the lean mixture, such as a cracked vacuum hose or weak fuel supply.
- Replace the upstream oxygen sensor if it does not respond to enrichment.
- Clear the codes and watch the sensor signal and fuel trims to confirm they return to normal.
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 or small leak keeps the cost low, while replacing the sensor or fixing a real lean condition raises it.
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 always mean the sensor is bad?
No. The sensor may be telling the truth about a lean mixture or air from an exhaust leak, so check those causes before replacing it.
How can I tell a bad sensor from a real lean condition?
If the sensor voltage rises quickly when the mixture is enriched, the sensor is fine and the problem is the mixture or a leak; if it stays low, the sensor or its circuit is at fault.
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