P0151: O2 Sensor Circuit Low Voltage, Bank 2 Sensor 1 (Meaning, Causes, Fixes)
What P0151 means
Bank 2 Sensor 1 is the upstream oxygen sensor, in front of the catalytic converter, on the side of the engine that does not contain cylinder 1. Only V6, V8 and other engines with two cylinder banks have a bank 2. A conventional oxygen sensor's voltage swings above and below about 450 millivolts as the exhaust goes rich and lean, and the engine computer uses those swings to adjust the fuel mixture. When the voltage stays low, the computer sees what looks like a constantly lean exhaust on that bank. The two sources describe the trigger a little differently: OBD-Codes.com says P0151 sets when the sensor's voltage stays low for longer than 2 minutes, while RepairPal says it sets when there is a sustained low voltage signal for a specified amount of time, and describes the code as tracking how long the oxygen or air-fuel ratio sensor takes to start switching around the 450-millivolt point once the system goes into closed loop. P0151 is the bank 2 version of P0131. OBD-Codes.com also calls it essentially the same as P0136 and P0137, but those are codes for the downstream sensor (Sensor 2) on bank 1, so the closest match is P0131.
Severity: can I keep driving?
Drive with caution, fix soon
Usually drivable for now, but get it checked soon. RepairPal rates P0151 Moderate: not an immediate threat to safety or the car, but it should be diagnosed within a reasonable time. Both sources say there may be no symptoms beyond the check engine light, though RepairPal adds that emissions are often elevated and fuel economy can suffer. The front oxygen sensor is what the engine uses to set its fuel mixture on that side of the engine, so a fault here can mean a failed emissions test.
Common symptoms
- Check engine light on
- Often no other noticeable symptoms (both sources)
- Higher emissions (RepairPal)
- Worse fuel economy (RepairPal)
Likely causes
Roughly most to least common, based on the sources below. Your vehicle may differ.
- Faulty oxygen sensor (both sources)
- Wiring or circuit problem: OBD-Codes.com lists a short to voltage on the signal circuit and high resistance or an open in the signal circuit; RepairPal lists faulty oxygen sensor wiring or connections
- A problem with another powertrain component (RepairPal, which does not name the part)
- Vacuum leak or faulty mass airflow sensor, which RepairPal says are commonly misdiagnosed as a bad oxygen sensor
Diagnostic steps you can do
Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.
- Read all stored codes and freeze-frame data. If P0151 shows up with lean codes such as P0174, or with other oxygen sensor codes, sort those out together rather than replacing the sensor straight away.
- Find the Bank 2 Sensor 1 sensor (before the catalytic converter, on the side of the engine opposite cylinder 1) and inspect its wiring and connector for melted insulation, chafing against the exhaust, corrosion or a loose plug.
- Check for vacuum and intake leaks, such as cracked hoses or a loose intake duct, since RepairPal says these are sometimes mistaken for a bad sensor.
- With a scan tool showing live data, compare the bank 2 front sensor with the bank 1 front sensor on a warmed-up engine. A conventional sensor should switch up and down repeatedly; one that sits low while the other bank switches normally points to that sensor or its circuit.
Typical fixes
- Replace the faulty Bank 2 Sensor 1 oxygen sensor (both sources)
- Repair a short, open or high resistance in the sensor's signal circuit or connector (both sources)
- Fix a vacuum leak or a faulty mass airflow sensor if testing finds one (RepairPal lists these as common misdiagnoses)
Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.
🛠 When to call a pro
- The code comes back after a new sensor is fitted, which suggests wiring, a leak or another component
- You need a smoke test for vacuum leaks, mass airflow sensor testing, or a scope to check the sensor signal
- The car has a wide-band air-fuel ratio sensor in the Sensor 1 position, which RepairPal says this code can also apply to
Where to buy
Plain search links. Always match parts to your exact year, make, model and engine.
FAQs
Can I drive with P0151?
Usually, for a short while. RepairPal rates it Moderate, and both sources say there may be no symptoms other than the check engine light. Get it diagnosed soon, since emissions and fuel economy can suffer.
Does P0151 always mean a bad oxygen sensor?
No. Both sources also list wiring and connection problems, and RepairPal says a vacuum leak or a faulty mass airflow sensor is sometimes misdiagnosed as a bad oxygen sensor. Check those before buying a sensor.
What's the difference between P0151 and P0131?
They are the same fault on different sides of the engine. P0131 is the front oxygen sensor on bank 1 (the side with cylinder 1), and P0151 is the front oxygen sensor on bank 2.
How much does it cost to fix P0151?
Neither source gives a repair cost for P0151. It depends on whether the fix is a new sensor, a wiring repair or fixing a leak, so get it diagnosed first.
Sources
Last reviewed 2026-10-23. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.