P0131 and P0171 both showed up. Which do I fix first?

Fix the lean condition first. P0171 confirms the engine is actually lean. That means the sensor in P0131 may be reporting correctly. Look for vacuum leaks, fuel pressure issues, or MAF problems before buying an O2 sensor.

Is a low-voltage O2 sensor always lean, or could it be something else?

A stuck-low sensor can look exactly like a lean exhaust. The difference is that a stuck sensor shows no switching activity at all. A truly lean engine will show the sensor switching but spending most of its time below 0.45V.

Can I drive until my next oil change to fix P0131?

A few weeks is usually fine. The ECM compensates with positive fuel trim. But if you are also seeing lean misfires or P0171, address it sooner.

Codes › P0131

P0131: The upstream oxygen sensor on the left side is reading lean all the time

O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)

Moderate severityCan you keep driving?

Yes, short distances. The ECM will try to compensate by adding fuel, but fuel trims will hit their limit. Extended lean running can cause misfires and catalyst damage over time.

What it means

P0131 means the upstream oxygen sensor on bank 1 is producing a signal stuck below 0.45 volts. A low voltage reading means the sensor sees excess oxygen in the exhaust, indicating a lean condition. But the more common cause is a sensor that is simply not switching and is stuck low, rather than a truly lean exhaust. A sensor stuck low produces the same code as a genuine lean exhaust condition, which is why you should check fuel trims before assuming the sensor is bad.

Common causes

Failed O2 sensor stuck at low voltageThe most common cause. The sensor has worn and simply stays low instead of switching.
Genuine lean exhaust conditionA vacuum leak, weak fuel pressure, or clogged injectors actually leans out the mixture and the sensor reports correctly.
Exhaust leak before the sensorOutside air entering the exhaust upstream of the sensor makes it see excess oxygen and report lean.
Broken signal wire or poor groundAn open signal circuit floats toward low voltage, producing the same reading as a stuck-lean sensor.

What it costs

Upstream O2 sensor replacement$150 to $380
Exhaust leak repair$100 to $400
Vacuum leak repair$50 to $300

Symptoms

Check engine light on
Long-term fuel trims significantly positive (engine adding fuel to compensate)
Lean stumble on light acceleration
Possible exhaust popping on deceleration
P0171 lean code may appear alongside

What to do next

Check long-term fuel trims before replacing the sensor. If LTFT is above plus 10 percent, the engine is genuinely lean and replacing the sensor will not fix the root cause. Look for vacuum leaks, low fuel pressure, or a dirty MAF sensor first. If fuel trims are close to zero and only the sensor reads stuck low, replace the sensor. An exhaust leak before the sensor is a common overlooked cause of a stuck-low reading.

Lewis shows the upstream O2 voltage and fuel trim values together so you can tell whether this is a lean engine or just a stuck sensor. Download Lewis.

Related codes

FAQ

P0131 and P0171 both showed up. Which do I fix first?: Fix the lean condition first. P0171 confirms the engine is actually lean. That means the sensor in P0131 may be reporting correctly. Look for vacuum leaks, fuel pressure issues, or MAF problems before buying an O2 sensor.
Is a low-voltage O2 sensor always lean, or could it be something else?: A stuck-low sensor can look exactly like a lean exhaust. The difference is that a stuck sensor shows no switching activity at all. A truly lean engine will show the sensor switching but spending most of its time below 0.45V.
Can I drive until my next oil change to fix P0131?: A few weeks is usually fine. The ECM compensates with positive fuel trim. But if you are also seeing lean misfires or P0171, address it sooner.