# P0157: O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2) **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-$450 - **Urgency:** Keep driving - **Most likely:** Failed rear O2 sensor or an exhaust leak - **DIY?:** Steps 1-4 ## Quick answer P0157 means the downstream oxygen sensor on bank 2 is reporting a voltage that stays too low, as if the exhaust after the catalytic converter were always lean. Often the sensor itself has failed, but an exhaust leak letting in outside air, a wiring short or a real lean condition can cause the same reading. ## What does P0157 mean? The downstream oxygen sensor on bank 2 sits after the catalytic converter on the side of a V6 or V8 that does not have cylinder 1. A conventional sensor makes its own small voltage: around 0.1 volt when the exhaust has extra oxygen (lean) and up to about 0.9 volt when it has little (rich). Behind a working converter, this sensor normally holds a fairly steady reading somewhere in the middle to upper part of that range. P0157 sets when the voltage stays below the maker's low limit for a set time while the engine is warm and running in closed loop. Some makers make the test active by briefly richening the mixture, or by watching the sensor during heavy acceleration, and expecting the voltage to rise; if it never does, the code sets. Most cars need the fault on two drive cycles. A real lean mixture and a sensor that simply cannot produce voltage look the same from the computer's side. ## What are the symptoms of P0157? - Check engine light on, usually with no change in how the car drives - Catalyst monitor not ready for an emissions test - Ticking or puffing from the exhaust on bank 2 if a leak is the cause - Lean code P0174 at the same time if bank 2 is truly running lean ## What causes P0157? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Failed downstream oxygen sensor** (most common). An old or contaminated rear sensor can lose the ability to produce voltage, so it reads near zero no matter what the exhaust is doing. This is common on high-mileage vehicles and after the sensor has been soaked by water or damaged by heat. If the voltage stays flat while the engine is clearly running rich, the sensor is bad. 2. **Exhaust leak ahead of the rear sensor** (common). A leak between the engine and the bank 2 rear sensor lets outside air into the exhaust between pulses. The sensor sees that extra oxygen and reports lean, so its voltage stays low even though the engine is fine. Rusted flanges, cracked flex pipes and loose clamps are typical culprits, especially on older trucks and cars in salt states. 3. **Signal wire shorted to ground** (less common). If the sensor's signal wire rubs through its insulation against the body, a bracket or the exhaust, the voltage is pulled toward zero. This often follows exhaust or transmission work that left the harness hanging. Unplugging the sensor and checking the harness side for continuity to ground tells a shop whether the wire is shorted. 4. **Real lean condition on bank 2** (less common). A vacuum leak, weak fuel injector or low fuel pressure can make bank 2 run genuinely lean, and the rear sensor correctly reports it. Positive fuel trims on bank 2 and a lean code such as P0174 support this. In that case, replacing the sensor will not help; the mixture problem has to be found and fixed first. ## How do you diagnose P0157? 1. Read all codes and fuel trims; high positive bank 2 trims point to a real lean condition. 2. Watch the bank 2 rear sensor on a scan tool and snap the throttle or add a little propane to see if the voltage rises. 3. Look and listen for exhaust leaks between the engine and the rear sensor. 4. Unplug the sensor and check the signal wire for a short to ground. 5. Replace the sensor only after leaks, wiring and fuel trims check out. ## How do you fix P0157, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace bank 2 downstream oxygen sensor | $120-$350 | Most are reached from under the vehicle. | | Repair exhaust leak | $100-$450 | A gasket or clamp at the low end, a flex pipe or pipe section higher. | | Repair shorted sensor wiring | $100-$250 | Repair the insulation or splice, then secure the harness. | Full price breakdowns: [oxygen sensor replacement cost](/cost/oxygen-sensor-replacement), [muffler replacement cost](/cost/muffler-replacement). ## Is it safe to drive with P0157? > **Safe to drive.** Yes, it is usually safe to drive. If P0157 comes with a lean code, rough running or hesitation, have the lean condition checked soon, because running lean for a long time can cause misfires and heat damage. ## Will P0157 fail an emissions test? P0157 turns on the check engine light, which fails an OBD-based emissions test in most states. While the rear sensor reads low, the computer cannot judge the bank 2 catalytic converter, so the catalyst monitor often stays incomplete. Clearing the code resets all monitors, so retest only after several days of normal driving. ## After the repair: clearing P0157 Clear the code and drive for a few days, including some steady highway miles and a few firm accelerations on a warm engine. On a scan tool, the bank 2 rear sensor should rise above about 0.5 volt when you accelerate and then settle at a steady value. When the catalyst and oxygen sensor monitors complete with no pending code, it is fixed. ## How to prevent P0157 - Fix exhaust leaks as soon as you hear ticking or puffing under the car. - Keep the rear sensor harness clipped clear of the exhaust and driveshaft. - Treat lean codes like P0174 promptly so a mixture problem is not hidden. - Avoid deep water, which can crack a hot sensor element. - Have rusted exhaust flanges replaced before they leak air near the sensor. ## Codes often seen with P0157 - [P0156](/codes/p0156): O2 Sensor Circuit (Bank 2 Sensor 2) - [P0158](/codes/p0158): O2 Sensor Circuit High Voltage (Bank 2 Sensor 2) - [P0174](/codes/p0174): System Too Lean (Bank 2) - [P0430](/codes/p0430): Catalyst System Efficiency Below Threshold (Bank 2) - [P0131](/codes/p0131): O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1) ## Frequently asked questions ### Can I drive with a P0157 code? Yes, P0157 is usually safe to drive with while you plan a repair. The rear sensor does not control fueling much, so the car normally runs the same. If you also have lean codes, rough idle or hesitation, get it checked sooner, because a real lean condition can lead to misfires. ### What does low voltage mean on an oxygen sensor? Low voltage means the sensor is reporting lean exhaust, with more oxygen than expected. Conventional sensors read about 0.1 volt when lean and up to 0.9 volt when rich. A reading stuck low can come from a dead sensor, outside air leaking in, a shorted wire or a truly lean engine. ### How much does it cost to fix P0157? Most P0157 repairs cost $100 to $450 at an independent shop. A new downstream oxygen sensor runs about $120 to $350 installed. Exhaust leak repairs cost $100 to $450, and fixing a shorted wire usually runs $100 to $250. Diagnosis fees are often around one hour of labor. ### Can an exhaust leak cause P0157? Yes, a leak upstream of the rear sensor is one of the most common causes. Outside air enters between exhaust pulses, adding oxygen, so the sensor reads lean. You may hear ticking or puffing, mostly on a cold start. Repair the leak first, then see whether the code returns before buying a sensor. ### Is P0157 a bad catalytic converter? Not usually. A worn converter tends to make the rear sensor switch up and down like the front one, which sets P0430, rather than holding it low. P0157 points to the sensor, wiring, a leak or a lean mixture. Fix it first, then let the catalyst monitor run. ### What is the difference between P0137 and P0157? Both mean a downstream oxygen sensor voltage is stuck low. P0137 is the rear sensor on bank 1, the side with cylinder 1, and P0157 is the rear sensor on bank 2. If both appear together, look for something shared, such as an exhaust leak near a merge point or a fueling problem. ### Can I fix P0157 myself? You can do much of it yourself. Checking for exhaust leaks, inspecting the wiring and replacing an easy-to-reach rear sensor with an oxygen sensor socket are reasonable home jobs. Testing fuel trims and wiring shorts is easier with a scan tool and multimeter. Leak repairs that need welding are shop work. ### Will P0157 pass inspection? No, with the check engine light on, the car fails an OBD-based emissions inspection in most states. The catalyst monitor for bank 2 may also be incomplete. After the repair, drive normally for several days, including steady highway cruising, and confirm the monitors read ready before you go for the test. ### Will a new oxygen sensor fix P0157? Only if the old sensor is actually bad. An exhaust leak, a shorted wire or a lean-running bank 2 will make a brand-new sensor read low as well. Watch the sensor on a scan tool while richening the mixture; if it never rises, the sensor or its wiring is at fault. ### Does P0157 hurt fuel economy? Usually only a little. Some cars use the rear sensor to fine-tune fuel, so a stuck-low reading may make the computer add slightly more fuel than needed. A noticeable drop in mileage suggests something else, such as a lean condition, a vacuum leak or an upstream sensor problem, also needs attention. ## 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 - [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) - [P0151 O2 Sensor Circuit Low Voltage (Bank 2 Sensor 1)](https://diagxcar.com/codes/p0151) - [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) - [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) - [P0171 System Too Lean (Bank 1)](https://diagxcar.com/codes/p0171) - [P0172 System Too Rich (Bank 1)](https://diagxcar.com/codes/p0172) ## Repair costs - [Oxygen sensor replacement cost](https://diagxcar.com/cost/oxygen-sensor-replacement): $150-$450 - [Muffler replacement cost](https://diagxcar.com/cost/muffler-replacement): $200-$600 --- Source page: https://diagxcar.com/codes/p0157 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.