P0157 on a 2012 Subaru Forester
Post-Cat O2 Sensor Low Voltage (Bank 2 Downstream)
P0157 on a 2012 Subaru Forester indicates post-cat o2 sensor low voltage (bank 2 downstream). It usually stays drivable short-term but should be diagnosed promptly. The most common cause is bank 2 downstream o2 signal wire shorted to ground or open (typically $80–$350). Confirm the root cause before replacing parts.
What does P0157 mean on a 2012 Subaru Forester?
P0157 is the Bank 2 counterpart of P0137. The downstream oxygen sensor on the cylinder bank that does NOT contain cylinder number 1 is reporting a voltage below the calibrated minimum — typically under about 0.1 V — for an extended period. Sensor 2 sits after the catalytic converter, so on a warm engine it should hold a relatively steady 0.6–0.8 V rather than switching. A reading pinned low points at an open or shorted-to-ground signal circuit, a failed sensor, fresh air entering the exhaust ahead of the sensor, or a genuine lean condition on that bank.
The symptoms, causes, and diagnostic steps below describe P0157 itself, so they read the same on nearby Subaru Forester model years. Powertrain and trim details vary year to year — confirm the specifics against your own vehicle before buying parts.
Is it safe to drive a 2012 Subaru Forester with P0157?
In most cases a 2012 Subaru Forester stays drivable for short trips with P0157 active, but diagnose and repair it promptly. This is a moderate-severity code — ignoring it can lead to further damage or a failed emissions test.
What are the symptoms of P0157 on a 2012 Subaru Forester?
- Check Engine Light is illuminated
- Catalyst readiness monitor will not complete
- Often no drivability symptoms at all
- Possible slight reduction in fuel economy
- Vehicle will fail emissions / smog testing
- Sometimes accompanied by P0174 if Bank 2 is genuinely lean
What causes P0157 on a 2012 Subaru Forester?
| Cause | Likelihood | Estimated repair (USD) |
|---|---|---|
| Bank 2 downstream O2 signal wire shorted to ground or open | Most common | $80–$350 |
| Failed Bank 2 downstream O2 sensor (stuck low) | Most common | $150–$450 |
| Exhaust leak ahead of the Bank 2 downstream sensor drawing in fresh air | Common | $100–$500 |
| Corroded Bank 2 downstream O2 sensor connector | Common | $30–$200 |
| Damaged sensor pigtail from a previous exhaust or suspension repair | Occasional | $80–$300 |
| Genuine lean condition on Bank 2 from a vacuum leak A Bank-2-only lean condition usually means an intake leak specific to that bank. | Occasional | $80–$600 |
| Failed PCM input for the Bank 2 downstream circuit | Rare | $400–$1,500 |
Cost ranges are editorial estimates for parts and labor at an independent U.S. shop — not quotes. What you actually pay moves with your region, the shop's hourly rate, which engine you have, and how much diagnostic time the job takes. Dealer service departments typically sit above these ranges.
How to diagnose this on a 2012 Subaru Forester
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Identify Bank 2, then identify Sensor 2
Two chances to go to the wrong part here. Bank 2 is the bank without cylinder number 1 — confirm which physical side that is in service information, because it varies by manufacturer. Sensor 2 is the sensor AFTER the catalytic converter on that bank, not the one in the manifold. Get either wrong and you replace a healthy sensor.
Tools: Vehicle-specific service information, Vehicle lift or jack stands
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Compare Bank 2 and Bank 1 fuel trims
This is the fastest way to tell a lying sensor from a real problem. If Bank 2 long-term fuel trim is near zero, the engine is fuelling correctly on that bank and the sensor or its wiring is at fault. If Bank 2 trim is high positive while Bank 1 is normal, you have a genuine bank-specific lean condition — find the leak rather than replacing the sensor.
Tools: Scan tool with per-bank fuel trim PIDs
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Disconnect the sensor and check for bias voltage
Unplug the Bank 2 downstream sensor and turn the key on. The scan tool PID should rise to the ECM's bias voltage, typically around 0.4–0.5 V. If it stays pinned near zero with the sensor disconnected, the signal wire is shorted to ground between the connector and the PCM, and no sensor will fix that.
Tools: Scan tool, Multimeter
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Inspect the harness under the vehicle
Follow the wiring from the sensor to the chassis connector. Look for insulation melted by exhaust contact, chafe points against the underbody, and corrosion or water inside the connector. On many V engines the Bank 2 downstream harness takes a longer route across the underside of the car, giving it more places to fail.
Tools: Flashlight, Inspection mirror, Electrical contact cleaner
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Look for exhaust leaks between the Bank 2 catalyst and sensor
A pinhole, a cracked weld, or a leaking flange gasket ahead of the sensor lets fresh air into the stream, and the sensor honestly reports the oxygen it finds — as a permanently low voltage. Listen along the pipe with the engine cold, when leaks are loudest before the metal expands.
Tools: Mechanic's stethoscope, Smoke machine (optional), Flashlight
NHTSA complaints & recalls for the 2012 Subaru Forester
Owner-reported safety complaints and official recalls filed with the U.S. National Highway Traffic Safety Administration for the 2012 Subaru Forester. Use these to gauge how common a problem is on your specific vehicle before you start chasing Subaru Forester diagnostics.
- ENGINE 27
- VEHICLE SPEED CONTROL 18
- AIR BAGS 20
- SERVICE BRAKES 15
- ELECTRICAL SYSTEM 10
15 active recalls
- AIR BAGS:FRONTAL:PASSENGER SIDE:INFLATOR MODULE Jan 2019
Subaru of America, Inc. (Subaru) is recalling certain 2010-2014 Tribeca, WRX, Outback, and Legacy vehicles, 2010-2011 Impreza vehicles and 2010-2013 Forester vehicles sold, or ever registered, in the states of Arizona, Arkansas, Delaware, District of Columbia, Illinois, Indiana,…
NHTSA campaign 19V008000 - AIR BAGS:FRONTAL:PASSENGER SIDE:INFLATOR MODULE Jan 2019
Subaru of America, Inc. (Subaru) is recalling certain 2010-2013 Forester, 2010-2014 Tribeca, WRX, Outback, Legacy, and 2010-2011 Impreza vehicles sold, or ever registered, in the states of Alabama, California, Florida, Georgia, Hawaii, Louisiana, Mississippi, South Carolina, Texa…
NHTSA campaign 19V007000 - AIR BAGS:FRONTAL:PASSENGER SIDE:INFLATOR MODULE Jan 2019
Subaru of America, Inc. (Subaru) is recalling certain 2010-2014 Tribeca, WRX, Legacy and Outback vehicles, 2010-2011 Impreza vehicles and 2010-2013 Forester vehicles sold, or ever registered, in the states of Alaska, Colorado, Connecticut, Idaho, Iowa, Maine, Massachusetts, Michi…
NHTSA campaign 19V009000 - ELECTRICAL SYSTEM:IGNITION:SWITCH Apr 2019
Subaru of America, Inc. (Subaru) is recalling certain 2009-2013 Forester vehicles, 2008-2011 Impreza vehicles, 2008-2014 WRX vehicles, 2005-2014 Legacy vehicles, 2005-2014 Outback vehicles and 2006-2008 Tribeca vehicles equipped with a mechanical key ignition switch installed as…
NHTSA campaign 19V297000
How do I fix P0157 on a 2012 Subaru Forester?
- Replace the Bank 2 downstream O2 sensor with an OEM part
- Repair the shorted-to-ground or open O2 signal wire
- Repair exhaust leaks between the Bank 2 catalyst and its downstream sensor
- Clean or replace the corroded sensor connector
- Repair Bank 2 vacuum leaks if fuel trims confirm a real lean condition
Engines in the 2012 Subaru Forester
The 2012 Subaru Forester was offered with the 2.5L 4-cylinder engine. Which one you have changes where P0157 is most likely to originate, so confirm your engine before ordering parts.
Engine data: EPA fueleconomy.gov.
Two chances to work on the wrong sensor
P0157 names a specific sensor in a specific place, and both halves of that name matter:
- Bank 2 — the cylinder bank without cylinder number 1. Which physical side that is depends on the manufacturer’s cylinder numbering. Look it up.
- Sensor 2 — the sensor after the catalytic converter, underneath the vehicle. Not the one in the exhaust manifold.
The wrong bank and the wrong sensor position are the two most common ways this repair gets done twice.
Fuel trim tells you whether to buy a sensor at all
Per-bank fuel trim is the deciding data point, and reading it takes less time than jacking the car up:
- Bank 2 trim near zero → the engine is fine, the sensor or its wiring is lying. Test the circuit, then replace the sensor.
- Bank 2 trim high positive, Bank 1 normal → the engine really is lean on that bank only. That pattern points at an intake gasket, a cracked vacuum line, or an injector problem specific to Bank 2. The sensor is doing its job.
- Both banks high positive → something common to both, such as a weak fuel pump or a large intake leak. Again, not the sensor.
Bank 2 sensors are not automatically the same job as Bank 1
The sensor part is usually identical to its Bank 1 twin. The access can be very different. On transverse V6s the Bank 2 exhaust often routes around the back of the engine, and the sensor may sit above a subframe or behind a heat shield that has rusted into place. Before assuming the job is a repeat of the Bank 1 sensor you did last year, check the labour time for this specific position.
Why this does not affect how the car drives
The downstream sensor grades the catalyst; the upstream sensor runs the fuelling. A dead Bank 2 downstream sensor mostly costs you the catalyst readiness monitor and the emissions test that depends on it. There is no mechanical harm in driving the car, and there is also no chance of the fault clearing itself — the monitor will keep failing until the circuit is repaired.
P0157 on a 2012 Subaru Forester: frequently asked questions
What does diagnostic trouble code P0157 mean on a 2012 Subaru Forester?
P0157 is the Bank 2 counterpart of P0137. The downstream oxygen sensor on the cylinder bank that does NOT contain cylinder number 1 is reporting a voltage below the calibrated minimum — typically under about 0.1 V — for an extended period. Sensor 2 sits after the catalytic converter, so on a warm engine it should hold a relatively steady 0.6–0.8 V rather than switching. A reading pinned low points at an open or shorted-to-ground signal circuit, a failed sensor, fresh air entering the exhaust ahead of the sensor, or a genuine lean condition on that bank.
What are the symptoms of P0157 on a 2012 Subaru Forester?
Check Engine Light is illuminated. Catalyst readiness monitor will not complete. Often no drivability symptoms at all. Possible slight reduction in fuel economy. Vehicle will fail emissions / smog testing. Sometimes accompanied by P0174 if Bank 2 is genuinely lean
What causes P0157 on a 2012 Subaru Forester?
Bank 2 downstream O2 signal wire shorted to ground or open (most-common). Failed Bank 2 downstream O2 sensor (stuck low) (most-common). Exhaust leak ahead of the Bank 2 downstream sensor drawing in fresh air (common). Corroded Bank 2 downstream O2 sensor connector (common). Damaged sensor pigtail from a previous exhaust or suspension repair (occasional). Genuine lean condition on Bank 2 from a vacuum leak (occasional). Failed PCM input for the Bank 2 downstream circuit (rare)
Is it safe to drive a 2012 Subaru Forester with P0157?
In most cases a 2012 Subaru Forester stays drivable for short trips with P0157 active, but it should be diagnosed and repaired promptly — this is a moderate-severity code. Ignoring it can lead to further damage or a failed emissions test.