P0036 on 2011-2018 Subaru Forester 2.5L: Rear O2 Sensor Heater Circuit Causes and Fixes

What's Unique About the 2011-2018 Subaru Forester

On the 2.5L non-turbo Boxer engine, there is only one cylinder bank, so 'Bank 1' is the only bank. The key challenge for this repair is often not the diagnosis but the physical removal of the old sensor, which can seize in the exhaust pipe due to rust and heat cycles. The electrical connector is conveniently located on the passenger side of the engine bay, next to the air intake snorkel. A common and frustrating Subaru feature is that this code, like almost any Check Engine Light fault, will disable the cruise control system and other driver aids as a safety precaution.

Generation note: This range covers two Forester generations: the SH (2011-2013) and the SJ (2014-2018). Both generations used the 2.5L FB25 engine (non-turbo), so the causes, symptoms, and repair process for code P0036 are virtually identical between them. The location of the sensor and its connector in the engine bay is consistent across these years. Part numbers are interchangeable across this entire year range.

Symptoms You May Notice

• Check Engine Light is on
• Cruise control is disabled (a common Subaru feature when a CEL is present)
• Vehicle may fail an emissions test
• In some rare cases, a subtle change in fuel economy, but this is not typical for a downstream sensor issue.

Most Likely Causes

1. Failed Rear Oxygen Sensor 🔴 High Probability → Shop Oxygen Sensor The internal heater element is a common failure point on oxygen sensors after many years and thousands of heat cycles. It's a wear-and-tear item.
   How to confirm: Disconnect the sensor and measure the resistance between the two same-colored wires (usually black) leading into the sensor. An infinite resistance or 'OL' reading on a multimeter indicates the heater circuit is open and the sensor is bad. 🎬 See how to test and replace the rear oxygen sensor. A healthy heater should have a low resistance, typically between 5 and 15 ohms, with some sources citing values as low as 2 ohms or as high as 25 ohms being acceptable. A reading of 0 ohms indicates a short circuit, which also requires sensor replacement.
   Typical fix: Replace the rear (downstream) oxygen sensor.
   Est. part cost: $70-$220
2. Damaged Wiring or Connector ⚪ Low Probability The wiring harness runs near the hot exhaust system and can become brittle or damaged over time, though it is fairly well-protected on the Forester.
   How to confirm: Visually inspect the wiring harness from the sensor up to the connector in the engine bay for any signs of melting, chafing, or breaks. Check for corrosion or water intrusion in the connector itself. With the ignition on, check for 12-volt power at the harness-side connector on the appropriate pin (a wiring diagram is needed to be certain).
   Typical fix: Repair or replace the damaged section of the wiring harness. Clean or replace the connector if corroded.
   Est. part cost: $10-$50
3. Blown Fuse ⚪ Low Probability
   How to confirm: Check the fuse box in the engine compartment for a blown fuse. On this platform, it is often Fuse #13 (15A) in the main fuse box, labeled for the A/F and O2 sensors. A blown fuse often indicates a short circuit in the wiring or the sensor itself that must be found and repaired.
   Typical fix: Replace the blown fuse after diagnosing and fixing the underlying short circuit.
   Est. part cost: $1-$5

Rare But Worth Checking

• Faulty Engine Control Module (ECM): → Shop Engine Control Module (ECM) This is extremely rare. The ECM should only be considered after all other possibilities, including the sensor, wiring, and fuses, have been thoroughly ruled out with a multimeter.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0036 is the only code present.
2. Visually inspect the rear oxygen sensor's wiring harness for any obvious damage, such as melting or breaks, from the exhaust pipe up to the engine bay.
3. Locate Fuse #13 (15A) in the engine bay fuse box and check if it is blown.
4. If the fuse and wiring appear intact, disconnect the rear O2 sensor's electrical connector (the all-gray connector located on the passenger side of the engine bay).
5. Use a multimeter set to Ohms (Ω) to measure the resistance across the two heater pins on the sensor side of the connector (these are the two black wires).
6. If the multimeter reads infinite resistance (Open Loop) or zero (short), the heater inside the sensor has failed and the sensor must be replaced.
7. A good sensor should read between 5 and 15 ohms, though some service data suggests a tighter range near 2-6 ohms.
8. If the resistance is within spec, check for 12V power at the harness-side connector with the vehicle's ignition on (but engine off). If there is no power, a wiring or fuse issue is the primary suspect.
9. If all tests pass but the code persists, the issue may be an intermittent wiring fault or, very rarely, the ECM.

Parts You'll Likely Need

• Rear Oxygen Sensor (Downstream) (OEM #22690AA960) — This is the most common failure for code P0036, as the internal heater element burns out over time. Denso is the Original Equipment Manufacturer (OEM) for Subaru.
  Trusted brands: Denso (OEM), Bosch, NGK/NTK
  OEM price range: $150-$220
  Aftermarket price range: $70-$120

Technical Service Bulletins (TSBs) & Recalls

• While no TSBs directly address P0036 specifically, TSB 11-130-13R addresses other DTCs for the FB engine family and shows the level of detail Subaru provides for sensor-related diagnostics, though it is not directly relevant to fixing P0036.

Platform-Specific Known Issues

• Cruise Control Disabled by CEL: A well-documented behavior on Subarus is the disabling of cruise control, and often other systems like EyeSight and traction control, whenever any check engine light is active. Many owners are surprised by this, but it is intentional. Clearing the P0036 code by fixing the sensor will restore cruise control function.

Mechanic-Grade Diagnostic Values

• O2 Sensor Heater Element Resistance — expected: ~2 to 6 Ohms. Failure: Infinite resistance (Open/OL) or 0 Ohms (Short).
• O2 Sensor Heater Circuit Current Draw — expected: Typically 0.5 Amps. Failure: Significantly higher or lower current draw, which may point to a short or open circuit respectively.
• Heater Circuit Ground Path Resistance — expected: Less than 0.1 Ohms. Failure: Values above 0.1 Ohms imply a poor ground connection, which can cause intermittent voltage drops and trigger the code.

Scan Tool Commands That Help

• Subaru Select Monitor (SSM) or equivalent professional scanner: ECM Reprogramming/Calibration Update — Extremely rare for this code. Only to be considered if a software update specifically addressing heater circuit misreads has been released by Subaru and all hardware (sensor, wiring, grounds, fuse) has been proven to be good.

Wiring & Ground Locations

• Exhaust Ground Strap — A small braided metal strap connecting the exhaust heat shield to the vehicle's frame, often located just inches away from the rear O2 sensor.. This strap provides a critical ground path for the O2 sensor through its body. If it corrodes and breaks (common in salt-belt regions), the sensor can lose its ground, causing a heater circuit fault even with a brand new sensor. OEM Denso sensors often have a metal plate designed to contact the heat shield to utilize this ground.
• Ge-1 — Main engine ground point located at the top center rear of the engine.. This is a primary ground point for the engine controls system. While less likely to be the sole cause for a specific sensor code, ensuring it is clean and tight is good practice during any electrical diagnosis.
• E25 / B19 — The wiring diagram identifier for the rear oxygen sensor's electrical connector.. Knowing these IDs helps in locating the correct connector and pinouts on a factory wiring diagram. 'B19' is associated with earlier models (up to 2013 Forester XT) and 'E25' with later models (2014+ Forester XT), suggesting a possible connector housing change between generations.

Real Owner Repair Stories

• Reddit user on r/subaru (2021 Subaru (newer model, but same design principle)) — Persistent O2 sensor 'heating circuit' error code on cold starts.
  ❌ Tried (didn't work) Replaced rear O2 sensor with an aftermarket NTK brand sensor; code returned immediately., Replaced the sensor again with an OEM Denso brand sensor; code still returned.
  ✅ What actually fixed it The owner discovered a grounding strap running from the exhaust heat shield to the vehicle frame, located two inches from the O2 sensor, was completely corroded and severed. After replacing the broken strap with a simple copper wire, the code was resolved permanently. The user noted the OEM Denso sensor has a metal plate that contacts the heat shield, making this ground path essential.

When the Usual Fixes Don't Work

• In at least one documented case, replacing the rear O2 sensor, even with an OEM Denso part, did not fix the P0036 code. The actual cause was a completely corroded and severed exhaust-to-frame ground strap located near the sensor. The fix was to replace this inexpensive strap, after which the code was resolved. This highlights the importance of checking all ground paths, not just the wiring in the sensor's harness.

OEM Part Supersession History

• 22690AA960 → 22690AA96A — Standard part number revision or supplier change.
  Heads up: The parts are considered fully interchangeable.

Model Year Variations Within This Range

• 2011-2013 (SH) vs 2014-2018 (SJ): While the sensor itself (22690AA960) is the same, the harness-side electrical connector receptacle may have a different housing. Wiring diagrams identify the connector as 'B19' for the SH generation Forester XT and 'E25' for the SJ generation Forester XT. This does not affect the repair but is a notable physical difference.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Excessive Oil Consumption 🔴 High — Common on 2011-2014 models with the FB25 engine, less so on 2015+. Caused by faulty piston rings. Can start as early as 60,000 miles. (Ref: TSB 02-157-14R; led to a class-action lawsuit and an extended warranty (8yr/100k miles) for affected vehicles.)
• CVT Transmission Issues 🟠 Medium — Reports of shuddering, hesitation, or outright failure. Most common on 2014-2015 models but can affect the entire generation. (Ref: Subaru extended the CVT warranty to 10 years/100,000 miles for many models, including the Forester, due to customer complaints.)
• Premature Wheel Bearing Failure 🟠 Medium — A humming or grinding noise from the wheel area that changes with speed is a common complaint, sometimes occurring before 100,000 miles.
• Front O2 (A/F) Sensor Failure 🟡 Low → Shop Oxygen Sensor — The front air/fuel ratio sensor is also a common failure item, though it produces different codes (e.g., P0171). (Ref: There was a recall for some early models related to the front O2 sensor.)
• Rear Suspension Spring Failure 🟡 Low — Some reports of rear coil springs breaking, particularly on 2014 models. (Ref: A recall was issued for some Foresters for faulty rear suspension springs.)

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For an oxygen sensor, a used part is almost never a smart choice. It is a wear-and-tear component with a finite lifespan. The labor to install it is the same, and the risk of premature failure is high. Only consider if it's from a verified, extremely low-mileage wreck and is significantly cheaper than a new part.

Donor-vehicle mileage cap: roughly under 20000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Verify the donor vehicle's mileage is low.
• Inspect the part for physical damage, heavy corrosion, or oil contamination.
• Ensure the electrical connector is clean and free of bent or corroded pins.

OEM-only on this vehicle (don't cheap out):

• Oxygen Sensor: While not strictly 'OEM-only', forum consensus strongly indicates that using the OEM supplier (Denso) is the most reliable path to avoid compatibility issues.

Aftermarket brands forum-validated for this vehicle:

• Denso (OEM supplier)

Brands owners have reported issues with on this vehicle:

• Generic/unbranded sensors from eBay or Amazon: These are frequently cited as being poor quality, failing quickly, or being out of spec right out of the box.
• Bosch: While a major brand, some Subaru owners have reported compatibility issues and premature failures with Bosch sensors in their vehicles.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0037 — Related heater circuit code (Low) for the same Bank 1 Sensor 2 location; often diagnosed using the same resistance testing procedure.
• P0038 — Related heater circuit code (High) for the same Bank 1 Sensor 2 location; shares the same diagnostic steps and common failure points.
• P0141 — A companion code specifically for the O2 Sensor Heater Circuit (Bank 1 Sensor 2), frequently appearing alongside or instead of P0036 when the heater element fails.
• P0171 — A lean condition code often associated with the front A/F sensor, which is another common failure item on this vehicle platform.

Frequently Asked Questions