P0136 on a 2012 Subaru Forester
Post-Cat O2 Sensor Circuit Fault (Bank 1)
P0136 on a 2012 Subaru Forester indicates post-cat o2 sensor circuit fault (bank 1). It usually stays drivable short-term but should be diagnosed promptly. The most common cause is failed bank 1 downstream o2 sensor (typically $150–$450). Confirm the root cause before replacing parts.
What does P0136 mean on a 2012 Subaru Forester?
P0136 is set when the ECM detects a general circuit fault on the Bank 1 downstream (post-catalyst) oxygen sensor. Unlike the more specific P0137 (low voltage), P0138 (high voltage), or P0140 (no activity), P0136 is the generic circuit code that covers wiring issues, intermittent connection problems, or signals that go outside expected ranges in a way that doesn't fit a more specific fault category.
This guide covers P0136 across the 2010-2014 Subaru Forester generation — the symptoms, causes, and diagnostic steps below apply to every model year from 2010 through 2014.
Is it safe to drive a 2012 Subaru Forester with P0136?
In most cases a 2012 Subaru Forester stays drivable for short trips with P0136 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 P0136 on a 2012 Subaru Forester?
- Check Engine Light is illuminated
- Catalyst readiness monitor will not complete
- Possible slight reduction in fuel economy
- Often no drivability symptoms
- Vehicle will fail emissions / smog testing
What causes P0136 on a 2012 Subaru Forester?
| Cause | Likelihood | Estimated repair (USD) |
|---|---|---|
| Failed Bank 1 downstream O2 sensor | Most common | $150–$450 |
| Damaged sensor wiring (chafed against exhaust) | Common | $80–$350 |
| Corroded O2 sensor connector | Common | $30–$200 |
| Exhaust leak upstream of the downstream sensor | Occasional | $100–$500 |
| Damaged or compressed sensor pigtail from a previous repair | Occasional | $80–$300 |
| Failed PCM input (rare) | Rare | $400–$1,500 |
How to diagnose this on a 2012 Subaru Forester
-
Read all O2-related codes to refine the picture
P0136 alone is generic. With P0137 it points at low voltage specifically; with P0138 at high voltage; with P0140 at no activity at all; with P0141 at the heater. Cross-referencing tells you whether the diagnostic should target wiring, sensor element, or heater.
Tools: Scan tool with full code retrieval
-
Inspect the sensor wiring
Trace the downstream sensor's harness from the sensor body back to the chassis connector. Look for melted insulation (from exhaust contact), chafing against suspension components, or rodent damage. Repair before replacing the sensor.
Tools: Flashlight, Inspection mirror, Wire repair supplies
-
Test the sensor connector
Disconnect and inspect for water intrusion, corrosion, bent pins. Clean with electrical contact cleaner. The downstream connector sits under the vehicle and gets road spray; corrosion is common after a few salt-belt winters.
Tools: Electrical contact cleaner, Magnifying glass
-
Graph the sensor signal
With engine warm and held at 2500 RPM, watch the Bank 1 downstream O2 voltage. A healthy sensor sits relatively steady around 0.6–0.8 V. Rapid switching, signal stuck at one value, or no signal at all confirms a sensor or wiring issue.
Tools: Scan tool with graphing PIDs
-
Verify the bias voltage from the PCM
Disconnect the sensor and key on. The scan tool should still show bias voltage (typically 0.4–0.5 V) at the connector. If not present, the issue is between the connector and the PCM, not the sensor.
Tools: Scan tool, Multimeter
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 P0136 on a 2012 Subaru Forester?
- Replace the Bank 1 downstream O2 sensor with an OEM part
- Repair damaged downstream sensor wiring
- Clean or replace corroded sensor connector
- Repair upstream exhaust leaks
About the 2010-2014 Subaru Forester
The 2010-2014 Subaru Forester was commonly sold with the following powertrains: 2.5L H4, 2.0L Turbo H4. Common trims include Base, Premium, Sport, Limited, Touring, Wilderness.
Why P0136 is the catch-all post-cat code
Each O2 sensor has a family of possible codes:
| Code | Meaning |
|---|---|
| P0136 | Generic circuit issue (catch-all) |
| P0137 | Voltage too low (open or short to ground) |
| P0138 | Voltage too high (short to power or rich condition) |
| P0140 | No activity detected (sensor isn’t reporting anything) |
| P0141 | Heater circuit fault |
When the ECM detects a problem that doesn’t fit the more specific patterns, it falls back to P0136. Reading other codes alongside narrows the diagnosis significantly — P0136 alone is much harder to pin down than P0136 + P0140.
What “downstream sensor” actually monitors
The Bank 1 downstream sensor is mounted in the exhaust after the catalytic converter on Bank 1 (the cylinder bank containing cylinder #1). Its main job is catalyst monitoring — comparing its signal to the upstream sensor’s signal to determine whether the catalyst is still scrubbing exhaust gases.
It provides a secondary, slower fuel-trim correction layer on top of upstream-sensor closed-loop control, but it is not the primary fuel feedback. A bad downstream sensor primarily affects emissions monitoring rather than drivability.
OEM vs aftermarket sensors
The downstream sensor’s role is comparison, not raw measurement, so it is slightly less picky about sensor response curve than the upstream. But aftermarket sensors still produce inconsistent results; for $30 more, an OEM Bosch, Denso, or NTK sensor is worth it on a vehicle you plan to keep.
P0136 on a 2012 Subaru Forester: frequently asked questions
What does diagnostic trouble code P0136 mean on a 2012 Subaru Forester?
P0136 is set when the ECM detects a general circuit fault on the Bank 1 downstream (post-catalyst) oxygen sensor. Unlike the more specific P0137 (low voltage), P0138 (high voltage), or P0140 (no activity), P0136 is the generic circuit code that covers wiring issues, intermittent connection problems, or signals that go outside expected ranges in a way that doesn't fit a more specific fault category.
What are the symptoms of P0136 on a 2012 Subaru Forester?
Check Engine Light is illuminated. Catalyst readiness monitor will not complete. Possible slight reduction in fuel economy. Often no drivability symptoms. Vehicle will fail emissions / smog testing
What causes P0136 on a 2012 Subaru Forester?
Failed Bank 1 downstream O2 sensor (most-common). Damaged sensor wiring (chafed against exhaust) (common). Corroded O2 sensor connector (common). Exhaust leak upstream of the downstream sensor (occasional). Damaged or compressed sensor pigtail from a previous repair (occasional). Failed PCM input (rare) (rare)
Is it safe to drive a 2012 Subaru Forester with P0136?
In most cases a 2012 Subaru Forester stays drivable for short trips with P0136 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.