P0131

O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)

Car Fault Codes Emissions & exhaust

Can you still drive? Yes, normally

Who can fix it? DIY with some experience

Typical repair time 30 minutes – 2 hours

Typical cost SAR 100 to 900

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.

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

0–500

500–2,000

2,000–5,000

5,000+

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

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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This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

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