P0131 Front O2 Sensor Low Voltage (Bank 1)
Get it checked soon
The front oxygen sensor on bank 1 is stuck reading low (lean). The sensor or its wiring has failed, or the engine really is that lean. Drivable, but fix it soon.
- Safe to drive?
- Yes, fix soon
- Typical cost at a shop
- $440-$523 RepairPal national averages, parts and labor
- Emissions test
- Yes, while the check engine light is on, most OBD emissions tests fail the car.
What it means
The front oxygen sensor on bank 1, the one before the catalytic converter, steers the fuel mixture. P0131 means it is stuck at a low reading, which looks like a very lean mixture. Either the sensor or its wiring has failed, or the mixture really is that lean.
The technical version
On cars with a narrow-band front sensor, P0131 sets when the signal stays below its low limit. On cars with a wide-band air-fuel sensor, the same code covers a low signal on that sensor's circuit, and the reading is a current or lambda rather than 0.1 to 0.9 V. The computer may fall back to fixed fueling while it is set.
Official name: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1).
Symptoms
- Worse fuel economy.
- Rough running on some cars.
- Often only the light.
Causes, most likely first
| Cause | How likely | How to tell |
|---|---|---|
| An old or fouled front oxygen sensor | most common | The sensor is original on a high-mileage car and reads flat. |
| Wiring chafed or shorted to ground, or a corroded connector | common | Melted insulation near the exhaust. |
| An exhaust leak near the sensor | less common | Ticking at the manifold on a cold start. |
| A real lean condition | less common | Fuel trims agree with the sensor, and P0171 may be set. |
How likely is our ranking from repair data and common practice, not a published rate.
What the live data shows
A healthy narrow-band front sensor sweeps between about 0.1 and 0.9 V and crosses the middle every second or two. With P0131 it sits low, under about 0.2 V. Check short-term fuel trim: it climbs as the computer adds fuel it thinks is missing. On a V engine, the bank 2 front sensor still switching normally points to bank 1's sensor.
What to check first, in order
-
Inspect the sensor wiring and connector.
Heat damage is common and cheap to fix.
What the data shows: No reading needed.
Cost: free to look
-
Check for an exhaust leak near the sensor.
Outside air makes it read lean.
What the data shows: Sensor stays low even when you add fuel.
Cost: free to look
-
Check fuel trims.
If trims agree, the engine is really lean.
What the data shows: Long-term trim high and positive: real lean. Near 0%: sensor.
Cost: no cost if you have a scanner
-
Replace the front oxygen sensor.
If wiring and leaks check out.
What the data shows: The sensor switches normally again.
Cost: $440-$523 at a shop (Oxygen sensor replacement, RepairPal)
Is it safe to drive?
Get it checked soon
Yes, fix soon. Fueling is off, which can harm the converter over time.
Drivable, but the computer is steering fuel with a bad signal. Expect worse fuel economy and maybe rough running. If lean codes like P0171 come with it, fix the lean condition soon.
Stop driving and get help if
- The check engine light flashes.
- The engine misfires.
What it costs
| Repair | Typical cost |
|---|---|
| Oxygen sensor replacementRepairPal, updated September 3, 2026 | $440-$523 |
National averages for the full job at a shop, parts and labor, from RepairPal (updated September 3, 2026). Your price depends on the car and the area. Many of the first checks on this page cost nothing with a scanner. Oxygen sensor replacement was CarMD's number 2 check engine repair in 2025, at an average of $287. CarMD averages come from a different method (repairs reported by its users), so they can sit below RepairPal's shop estimates.
Sources for the notes: CarMD 2026 Vehicle Health Index (press release, April 22, 2026).
Can I clear it? Will it come back?
After the repair, clear the code and drive. The oxygen sensor monitor should complete within a few drives.
Will it fail an emissions test?
Yes, while the check engine light is on, most OBD emissions tests fail the car.
Related codes
Also related, no guide yet: P0133, P0134, P0135.
How Obie handles P0131
Obie reads the front sensor, the fuel trims and the other bank, and tells you whether to suspect the sensor, its wiring or a real lean mixture. Obie is part of ScanMyCar Pro. Codes and live sensors are free.
Questions people ask
Is P0131 the upstream or downstream sensor?
Upstream: the front sensor on bank 1, before the catalytic converter.
Can I drive with P0131?
Yes, for a short time. Fix it soon to protect fuel economy and the converter.
Will a new O2 sensor fix P0131?
Often, but check the wiring and for exhaust leaks first.
Sources
- RepairPal: P0131 code page repairpal.com
- RepairPal: Oxygen sensor replacement (updated September 3, 2026) repairpal.com
- CarMD 2026 Vehicle Health Index (press release, April 22, 2026) prnewswire.com
- Texas DPS emissions testing FAQ dps.texas.gov
- Arizona myazcar.com: OBD and the check engine light myazcar.com
A guide, not a diagnosis. This page describes what P0131 generally means across OBD2 cars. What is actually wrong with your car depends on its live data and its history, and any repair decision deserves a proper diagnosis, whether that is you with a scanner or a professional with a lift.
