P0157 Code: O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
The O2 sensor after the bank 2 converter reads near zero volts – often a nearby exhaust leak or a worn sensor.
At a glance
Severity: Low
- Meaning
- O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $50–$450
- DIY difficulty
- Easy
P0157 means the sensor behind the bank 2 catalytic converter has stayed below a low threshold, typically under about 0.1 to 0.2 V, at times when the ECU expected it to rise. Behind a healthy converter this sensor normally sits in the 0.5 to 0.8 V range, because the converter uses up the leftover oxygen; a reading pinned near zero means the sensor either sees a lot of oxygen or isn't producing a signal. On bank 2 the usual reasons are an exhaust leak between the converter and the sensor, such as a rusted flange gasket, a cracked pipe or a loose clamp on that side of a dual or Y-pipe system, a sensor that has simply worn out, or a signal wire grounding against the underbody. Many ECUs use this sensor for a small fuel correction, so a false lean reading can make bank 2 run slightly rich over time.
Can I keep driving? Safe to drive. Performance is usually unaffected, but the bank 2 catalyst check can't be trusted and fuel use may creep up slightly if the ECU trims richer. Fix it before the next emissions test.
How to read P0157
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 57 Specific fault
Symptoms
- Check engine light, often after highway driving
- No noticeable drivability change in most cases
- Puffing or ticking sound under the car near the bank 2 converter
- Soot marks at a flange or clamp behind the converter
- Fuel economy a little lower than usual
- Exhaust smell under the car or in the cabin on some vehicles
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Exhaust leak between the bank 2 converter and the sensor: flange gasket, rusted pipe, loose clamp
- Bank 2 downstream sensor worn out and no longer producing voltage
- Signal wire rubbed through against the frame or heat shield and touching ground
- Genuinely lean bank 2 (usually also sets P0174 and upstream codes)
- Water in the connector pulling the signal toward ground
- ECU input fault or wrong replacement sensor
Diagnosis step by step
Compare both rear sensors at cruise
Graph bank 1 sensor 2 and bank 2 sensor 2 at a steady cruise on a warm engine. If bank 1 sits around 0.6 to 0.7 V and bank 2 hugs 0.05 V, the problem is confined to the bank 2 rear sensor, its wiring or the exhaust around it. Also check the bank 2 upstream sensor and fuel trim to confirm that bank 2 isn't genuinely lean.
Tools: OBD-II scanner
Do a hard-acceleration test
On a safe, open road, accelerate firmly from about 20 to 50 mph. Power enrichment floods the converter with rich exhaust and a working rear sensor must rise above about 0.8 V. If bank 1's rear sensor goes up and bank 2's stays under 0.2 V, the bank 2 sensor, its wiring or an air leak near it is at fault.
Tools: OBD-II scanner
Look for leaks around the sensor
With the exhaust cold, inspect the pipe from the bank 2 converter back past the sensor: flange gaskets, slip joints, clamps and rust holes. Black soot trails show where exhaust escapes, and on a pulsing exhaust the same gap pulls air in. A smoke machine fed into the tailpipe finds small leaks quickly.
Tools: Flashlight, Smoke machine
Test the signal wire for a short to ground
Unplug the sensor, disconnect the battery and measure resistance from the harness signal wire to chassis ground. It should read open. Any continuity means the insulation is damaged, often where the harness is clipped to the transmission or frame.
Tools: Multimeter
Check the sensor output directly
Back-probe the sensor's own signal and ground wires with the engine warm and repeat the acceleration test. If the voltage at the connector stays near zero, the sensor is dead; if it rises there but not on the scan tool, the fault is in the harness.
Tools: Multimeter
Replace and route carefully
Soak the sensor threads, which often rust badly under the car, and use an O2 socket on a warm pipe. Fit a direct-fit sensor at about 30 lb-ft and clip the pigtail back in its original holders away from the pipe and driveshaft.
Tools: O2 sensor socket, Torque wrench
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace flange gasket or clamp behind the bank 2 converter | No | $80–$250 |
| Replace bank 2 downstream O2 sensor yourself (part only) | Yes | $50–$180 |
| Replace bank 2 downstream O2 sensor at a shop | No | $150–$350 |
| Weld or replace a rusted pipe section | No | $120–$450 |
| Repair grounded sensor wiring | No | $80–$220 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
P0157 with the check engine light on fails an OBD-II smog or emissions inspection. An exhaust leak near the sensor can also affect a tailpipe test in states that still do one.
Frequently asked questions
Can P0157 hide a bad catalytic converter?
It can. The ECU judges the converter by comparing the front and rear sensors; a rear sensor stuck low looks calm and may make a weak converter seem fine. Once the sensor is fixed, a P0430 sometimes follows, which is a real finding rather than a new fault.
Why would an exhaust leak make the sensor read lean?
Exhaust flows in pulses, and between pulses the pressure at a small gap can drop enough to pull outside air in. That fresh oxygen reaches the sensor and makes it read lean even though the engine isn't.
Does P0157 mean my engine is running lean?
Usually not. If bank 2 were truly lean, the upstream sensor and fuel trims would show it and you'd typically see P0174 as well. A lone P0157 with normal upstream readings points to the rear sensor or its surroundings.
Is it worth replacing both rear sensors together?
Only if bank 1's rear sensor also fails the acceleration test or is visibly slow. Downstream sensors often last well, so replacing just the bank 2 one is usually enough.