OBD-II trouble code
P0157: O2 Sensor Circuit Low Voltage (Bank 2, Sensor 2)
The downstream oxygen sensor on bank 2 is reading persistently low — a steady lean-looking voltage that won't come up. As the bank-2 mirror of P0137, the usual suspects are an exhaust leak near the sensor letting in outside air, a genuine lean condition, or a failed sensor or wiring fault. Because this is a post-catalyst sensor, the driveability impact is usually small, but it can muddy catalyst monitoring.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $400
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0157 mean?
The downstream (sensor 2) oxygen sensor sits behind the catalytic converter, where its main job is to monitor catalyst efficiency rather than to trim fuel moment-to-moment. Behind a catalyst that is storing oxygen, the trace is steadier than the upstream sensor. No mid-range voltage window is stated for every downstream sensor. P0157 sets when the bank-2 downstream sensor's voltage stays abnormally low — pinned toward the lean end and not recovering. A low voltage corresponds to a lean or oxygen-rich reading, so the PCM is seeing either real extra oxygen in the exhaust at that point or a sensor/circuit that's reporting low when it shouldn't. 'Bank 2' is the cylinder bank without cylinder number one, and 'sensor 2' is the post-catalyst position.
The causes split between real exhaust conditions and sensor-circuit faults. An exhaust leak near the rear sensor is a classic cause: outside air drawn in reads as extra oxygen and holds the voltage low. A genuinely lean condition — a vacuum leak, low fuel pressure, or a lean-running bank 2 — can do the same, and the lean code for this bank is P0174, not P0171. On the circuit side, a failed sensor stuck low, a short to ground in the signal wire, a corroded connector, or a heater problem that keeps the element cold can all produce a persistent low reading. Because the downstream sensor isn't the primary fuel-control input, a low reading here is less about immediate driveability and more about whether the catalyst monitor can trust this sensor.
For the driver, P0157 usually means a check engine light with little or no change in how the car runs, though if a real lean condition is behind it you may notice a rough idle or hesitation. It will fail an emissions test and can interfere with catalyst-efficiency monitoring. Diagnosis starts by checking the bank-2 downstream sensor's live data and looking for an exhaust leak near it, then checking fuel trims for a real lean condition, and finally testing the sensor, heater, and wiring before replacing the sensor.
Common causes
- Exhaust leak near the bank-2 downstream sensor letting in outside air
- Genuine lean condition on bank 2 (vacuum leak, low fuel pressure)
- Failed downstream oxygen sensor stuck at low voltage
- Short to ground in the sensor signal wire
- Corroded or loose sensor connector
- Sensor heater fault keeping the element too cool
- Poor or corroded sensor ground offsetting the reported voltage
Symptoms
- Check engine light with P0157 stored
- Little or no change in driveability in most cases
- Possible rough idle or hesitation if a real lean condition is present
- Failed emissions test
- Steady low (lean) bank-2 downstream sensor reading
- Sometimes accompanied by the bank 2 lean code P0174
Diagnostic steps
- 1.View the bank-2 downstream O2 sensor live data and confirm the voltage stays abnormally low.
- 2.Inspect for an exhaust leak near the rear sensor — a common cause that draws in outside air and reads lean. Let the exhaust cool and put the vehicle on stands or a lift before going underneath; a leak is easiest to find with the engine running, but that inspection belongs to someone who can stay clear of hot pipework, and never under a car held up by a jack alone.
- 3.Check bank 2 fuel trim. The lean code for that bank is P0174. P0171 is bank 1 and does not locate this fault.
- 4.Inspect the sensor connector and signal wiring for a short to ground, corrosion, or resistance.
- 5.Verify the sensor heater circuit is functioning.
- 6.Replace the downstream sensor if leaks, lean conditions, and wiring all check out but the sensor stays pinned low.
Repair cost
$150 – $400
The low end of the printed bracket is an exhaust leak ahead of the bank 2 downstream sensor. The high end is the sensor, after a short to ground and a real bank 2 lean condition (P0174, not P0171) have been ruled out. Read the bracket as a planning span, not a quoted price.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with oxygen sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
Related repairs
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.