# P0151: O2 Sensor Circuit Low Voltage (Bank 2 Sensor 1) **Verdict:** Safe to drive Oxygen sensors - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $100-$600 - **Urgency:** Keep driving - **Most likely:** Failed bank 2 upstream O2 sensor or leak - **DIY?:** Steps 1-3 ## Quick answer P0151 means the upstream oxygen sensor on bank 2, ahead of that side's catalytic converter, is reading a voltage that stays too low. A low reading means the sensor thinks the exhaust is lean. It only appears on engines with two banks, such as V6 and V8s, and is often caused by a failed sensor or an exhaust leak on that side. ## What does P0151 mean? V6, V8 and flat engines have two rows of cylinders, called banks. Bank 1 is the side with cylinder 1 and bank 2 is the other side; which physical side that is depends on the engine, so check a service diagram. Each bank has its own upstream oxygen sensor, sensor 1, ahead of the catalytic converter, so the computer can trim fuel for each side separately. A narrow-band sensor switches between roughly 0.1 volt lean and 0.9 volt rich. P0151 sets when the bank 2 sensor 1 signal stays below a low threshold for too long when it should be switching, such as on a warm engine in closed loop. Some makers also check it during a commanded rich period. It usually needs the fault on two trips. Engines that use air-fuel ratio sensors upstream may report similar faults with different codes. P0151 is the bank 2 version of P0131. ## What are the symptoms of P0151? - Check engine light on - Car may run normally - Lower fuel economy - Ticking noise from one side of the engine on cold starts if a manifold leaks - Possible failed emissions test ## What causes P0151? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Failed bank 2 upstream oxygen sensor** (most common). An aging or contaminated sensor can get stuck low and stop switching. Upstream sensors run hot and see raw exhaust, so they wear faster than downstream ones, especially on engines that burn oil. A bank 2 sensor 1 signal sitting flat near zero on a warm engine, even after a throttle snap, points to the sensor. 2. **Exhaust manifold leak on bank 2** (common). V6 and V8 exhaust manifolds can crack or break their studs from repeated heating and cooling, and gaskets can blow out. Outside air drawn in ahead of the sensor reads as lean. This is common on high-mileage trucks and SUVs and in road-salt states. A ticking sound on one side of the engine on cold starts is a clue. 3. **Signal wire shorted to ground or damaged** (less common). Bank 2 sensor wiring is often routed near the back of the engine or along the transmission, where heat and movement can melt or chafe insulation. A signal wire shorted to ground pulls the voltage toward zero no matter what the exhaust is doing. Inspect the harness carefully after engine or transmission work. 4. **Real lean condition on bank 2** (less common). A vacuum leak, intake gasket leak or weak injectors on bank 2 cylinders can make that side truly lean, and the sensor is just reporting it. P0174 with P0151, plus high positive fuel trims on bank 2 only, points here. If both banks read lean, look for a shared cause such as the MAF sensor or fuel pressure. 5. **Engine computer fault** (rare). The engine computer reads the bank 2 sensor signal and supplies its bias. An internal fault can make a good sensor read low. This is uncommon and is normally considered only after the sensor, wiring and exhaust have tested good. A shop may check for software updates or service bulletins before replacing the computer. ## How do you diagnose P0151? 1. Read all codes; P0174 with P0151 suggests bank 2 is truly lean, while P0171 and P0174 together point to a shared cause. 2. Warm the engine and compare bank 2 sensor 1 with bank 1 sensor 1; both should switch quickly between about 0.1 and 0.9 volt on narrow-band sensors. 3. Snap the throttle or add propane; a good sensor should jump high, while a failed one stays low. 4. Listen for exhaust manifold leaks on the bank 2 side on a cold start, and look for broken studs. 5. Inspect the bank 2 sensor wiring for melted or chafed spots, and compare bank 2 fuel trims with bank 1. ## How do you fix P0151, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace the bank 2 upstream oxygen sensor | $150-$450 | Higher costs apply where the sensor sits near the firewall or needs parts removed. | | Repair a bank 2 exhaust manifold leak | $200-$900 | Gaskets and studs cost less; broken studs or a cracked manifold cost more. | | Repair the sensor wiring | $100-$300 | Fixes a signal wire shorted to ground or damaged by heat. | Full price breakdowns: [oxygen sensor replacement cost](/cost/oxygen-sensor-replacement), [exhaust manifold gasket replacement cost](/cost/exhaust-manifold-gasket-replacement). ## Is it safe to drive with P0151? > **Safe to drive.** It is usually safe to drive for a few weeks. The computer may add extra fuel to bank 2 based on the false lean reading, which can waste fuel and over time overheat the bank 2 converter, so do not put it off. ## Will P0151 fail an emissions test? P0151 keeps the check engine light on, which fails an OBD-II emissions test in most states. A bank 2 upstream sensor that reads low also stops the oxygen sensor monitor from passing and can keep the bank 2 catalyst test from running. After the repair and clearing the code, drive several days with some steady highway driving before testing. ## After the repair: clearing P0151 After the repair, clear the code. On a warm engine, bank 2 sensor 1 should switch as quickly as bank 1 sensor 1, and bank 2 fuel trims should settle near zero and close to bank 1. Drive several trips with steady cruising, then confirm the oxygen sensor monitor reads ready and no pending code has returned. ## How to prevent P0151 - Fix ticking exhaust manifold leaks early, before studs break and the repair grows. - Keep bank 2 sensor wiring clipped away from the hot manifold and transmission. - Repair oil burning and coolant leaks, which shorten oxygen sensor life. - Buy the sensor listed for bank 2 sensor 1, since it can differ from bank 1. ## Codes often seen with P0151 - [P0131](/codes/p0131): O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1) - [P0174](/codes/p0174): System Too Lean (Bank 2) - [P0153](/codes/p0153): O2 Sensor Circuit Slow Response (Bank 2 Sensor 1) - [P0155](/codes/p0155): O2 Sensor Heater Circuit (Bank 2 Sensor 1) - [P0430](/codes/p0430): Catalyst System Efficiency Below Threshold (Bank 2) - [P0171](/codes/p0171): System Too Lean (Bank 1) ## Frequently asked questions ### Can I drive with a P0151 code? Yes, P0151 is usually safe to drive with for a few weeks. The computer may add extra fuel to bank 2 because it thinks that side is lean, so expect lower mileage. Fix it soon to protect the bank 2 catalytic converter and to pass an emissions test. ### Where is bank 2 sensor 1? Bank 2 sensor 1 is the oxygen sensor ahead of the catalytic converter on the side of the engine without cylinder 1. On many V6 and V8 engines it sits in or just below the exhaust manifold. Which side that is varies by engine, so check a service diagram before buying or removing a sensor. ### How much does it cost to fix P0151? Most P0151 repairs cost $100 to $600 at an independent shop. A bank 2 upstream oxygen sensor runs about $150 to $450, depending on access. Wiring repairs run $100 to $300. An exhaust manifold leak costs $200 to $900, more if studs have broken off in the head. ### What is the difference between P0131 and P0151? They are the same fault on different sides of the engine. P0131 is low voltage from the upstream sensor on bank 1, the side with cylinder 1. P0151 is low voltage from the upstream sensor on bank 2. Four-cylinder and inline engines only have bank 1, so they cannot set P0151. ### Why do I have P0151 and P0174? P0174 means bank 2 is running lean, and P0151 means the bank 2 upstream sensor is reading low, which also looks lean. Together they often mean bank 2 is truly lean from a vacuum or exhaust leak on that side. Check for leaks before replacing the sensor. ### Can I replace the bank 2 oxygen sensor myself? Sometimes. On many trucks and some V6 cars, the bank 2 upstream sensor is reachable from below with an oxygen sensor socket. On some front-wheel-drive V6 cars it sits near the firewall and needs parts removed. Look at the access first, and use penetrating oil on rusted threads. ### Can an exhaust manifold leak cause P0151? Yes. A cracked manifold, broken stud or blown gasket on bank 2 lets outside air reach the upstream sensor, which reads it as lean and low. This is common on high-mileage V6 and V8 engines. A ticking sound on one side that fades once the engine warms is a typical sign. ### Should I replace both upstream sensors? Not unless both test bad. P0151 points only to bank 2 sensor 1. If both upstream sensors are original on a high-mileage engine, replacing them together can save a second labor charge later, but it is not needed to fix P0151 if bank 1 switches normally. ### Does P0151 hurt fuel economy? Often, yes. Because the bank 2 sensor reports lean, the computer may add fuel to that bank, and it cannot fine-tune the mixture accurately. Drivers may notice lower mileage. Once the sensor, leak or wiring is fixed, bank 2 fuel trims should return close to bank 1. ### Will P0151 damage my catalytic converter? It can over time. Extra fuel sent to bank 2 because of the false lean reading can overheat that side's converter, and a real lean condition can cause misfires that do the same. Fixing P0151 within a few weeks keeps the risk low, and is far cheaper than a converter. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other oxygen sensors codes - [P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0138) - [P0139 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0139) - [P0140 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0140) - [P0141 O2 Sensor Heater Circuit (Bank 1 Sensor 2)](https://diagxcar.com/codes/p0141) - [P0153 O2 Sensor Circuit Slow Response (Bank 2 Sensor 1)](https://diagxcar.com/codes/p0153) - [P0155 O2 Sensor Heater Circuit (Bank 2 Sensor 1)](https://diagxcar.com/codes/p0155) - [P0156 O2 Sensor Circuit (Bank 2 Sensor 2)](https://diagxcar.com/codes/p0156) - [P0157 O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)](https://diagxcar.com/codes/p0157) - [P0158 O2 Sensor Circuit High Voltage (Bank 2 Sensor 2)](https://diagxcar.com/codes/p0158) - [P0161 O2 Sensor Heater Circuit (Bank 2 Sensor 2)](https://diagxcar.com/codes/p0161) ## Symptoms - [Why is my car getting bad gas mileage?](https://diagxcar.com/symptoms/poor-gas-mileage) - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) ## Repair costs - [Oxygen sensor replacement cost](https://diagxcar.com/cost/oxygen-sensor-replacement): $150-$450 - [Exhaust manifold gasket replacement cost](https://diagxcar.com/cost/exhaust-manifold-gasket-replacement): $300-$1,300 --- Source page: https://diagxcar.com/codes/p0151 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.