P0138 on 2019-2023 Subaru Forester: O2 Sensor High Voltage Causes and Fixes

What's Unique About the 2019-2023 Subaru FORESTER

For this generation of Subaru Forester and its platform-mates (like the Crosstrek and Impreza), the P0138 code is a well-documented issue. Subaru acknowledged a problem with the original rear oxygen sensors and released Technical Service Bulletin #09-115-24R, which details the availability of a newly developed sensor to fix this specific code. The TSB explains the new sensors have an 'optimized coating application' to prevent internal cracking and short circuits, making this a known component issue rather than a random failure. The updated part is the recommended replacement.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is on
• Reduced fuel economy
• Rough or unstable idle
• Failure to pass an emissions test
• Strong exhaust odor

Most Likely Causes

1. Faulty Rear Oxygen Sensor (Bank 1, Sensor 2) 🔴 High Probability Subaru issued TSB #09-115-24R specifically to address this failure. The original sensors (part numbers like 22690AB070) are prone to internal cracking of the ceramic element, leading to a short circuit that causes a high voltage signal. The TSB introduced a more durable, updated sensor.
   How to confirm: Using an OBD-II scanner, observe the live data for 'Bank 1, Sensor 2' voltage. If it is stuck high (e.g., >0.9V) and does not fluctuate, the sensor has likely failed. A reading that stays consistently above 1.2V 🎬 Watch: How to diagnose and fix a P0138 high voltage code is a definitive confirmation of a failed sensor.
   Typical fix: Replace the rear (downstream) oxygen sensor with the updated OEM part (Subaru P/N 22690AC091) or a high-quality aftermarket equivalent like Denso P/N 234-4583.
   Est. part cost: $100-$250
2. Damaged Wiring or Connector 🟡 Medium Probability The sensor and its wiring are located under the vehicle and are exposed to road debris, moisture, and exhaust heat, which can lead to corrosion, frayed wires, or a short to a power source. The harness can also be damaged if not routed correctly away from the hot exhaust pipe.
   How to confirm: Visually inspect the entire wiring harness from the rear O2 sensor to the main connector. Look for melted insulation, corrosion in the connector pins, or breaks in the wire. Use a multimeter to check for a short to power on the signal wire.
   Typical fix: Repair the damaged section of wire or clean/replace the connector pigtail. Ensure the harness is secured in its clips away from the exhaust.
   Est. part cost: $10-$50

Rare But Worth Checking

• Engine Running Rich: This is less common for P0138 alone. A rich condition (too much fuel, not enough air) is usually caused by other issues like a leaking fuel injector or faulty Mass Airflow (MAF) sensor and will typically be accompanied by other trouble codes.
• Exhaust Leak: An exhaust leak *before* the rear O2 sensor could alter readings, but this is an unlikely cause for a high voltage reading, which indicates a lack of oxygen.
• Faulty Engine Control Module (ECM): An ECM failure is extremely rare and should only be considered after all other possibilities, including the sensor and wiring, have been exhaustively ruled out.

Diagnosis Steps

1. Connect an OBD-II scanner and confirm P0138 is present. Note any other codes.
2. Using the scanner's live data function, monitor the voltage for 'Bank 1, Sensor 2'. Confirm if it is stuck at a high voltage (above 0.9V). A reading above 1.2V confirms sensor failure.
3. Perform a thorough visual inspection of the rear oxygen sensor's wiring harness and connector, which is located on the exhaust pipe after the catalytic converter. The connector is often clipped to the transmission or frame rail. Look for any signs of melting, corrosion, or physical damage.
4. If the wiring appears intact, test the sensor's internal heater circuit resistance. A healthy sensor should read between 5 and 8 ohms. 🎬 See this guide on testing and replacing the rear oxygen sensor A reading over 50 ohms indicates a failed heater element, which necessitates sensor replacement.
5. Given the high probability of sensor failure confirmed by Subaru's TSB, if the wiring is good and/or the heater resistance is out of spec, replacing the rear oxygen sensor is the most logical step. A 22mm or 7/8-inch offset oxygen sensor socket will be required.
🎬 Watch: A step-by-step walkthrough of the downstream sensor replacement
6. After replacing the sensor, clear the trouble codes with the scanner and perform a test drive to ensure the code does not return. Disconnecting the battery for a few minutes will also clear the code and reset fuel trims.

Parts You'll Likely Need

• Rear Oxygen Sensor (Downstream) (OEM #22690AC091 (Updated Part)) — This is the most common failure point for code P0138 on this vehicle, as acknowledged by Subaru's TSB #09-115-24R. The original part (e.g., 22690AB070) is prone to failure.
  Trusted brands: Subaru (Genuine OEM), Denso (P/N: 234-4583), NGK/NTK
  OEM price range: $180-$250
  Aftermarket price range: $95-$160

Related Codes That Often Appear With This One

• P0137 — This code for 'O2 Sensor Circuit Low Voltage' is also covered by the same Subaru TSB (#09-115-24R) and relates to the failure of the same rear oxygen sensor, often due to an internal open or short.
• P0140 — This code for 'O2 Sensor Circuit No Activity Detected' is also mentioned in TSB #09-115-24R and points to a failure of the same component, often from a broken internal element.
• P0141 — This code for 'O2 Sensor Heater Circuit' is also part of the group of codes addressed by Subaru's TSB #09-115-24R for the rear O2 sensor, as a heater failure will prevent the sensor from working correctly.

Technical Service Bulletins (TSBs) & Recalls

• 09-115-24R: Announces an updated rear oxygen sensor to address DTCs P0137, P0138, P0140, and P0141 due to potential internal short circuits from cracking. This is the revised and most current bulletin.
• 09-115-24: The original version of the TSB announcing the new oxygen sensors for the same set of trouble codes.

Platform-Specific Known Issues

• Subaru has issued Technical Service Bulletins (TSB #09-115-24 and #09-115-24R) regarding the availability of a newly developed rear oxygen sensor to address codes P0137, P0138, P0140, and P0141. The TSB states the cause is potential internal cracking of the sensor element, and the new part has an 'optimized coating application' to improve durability.

Mechanic-Grade Diagnostic Values

• Rear O2 Sensor (B1S2) Live Data Voltage — expected: Slowly fluctuating between 0.1V and 0.9V on a warm, running engine.. Failure: Voltage is stuck high (above 0.9V). A reading consistently above 1.2V confirms a failed sensor.
• Rear O2 Sensor Heater Circuit Resistance — expected: 5 to 8 Ohms.. Failure: A reading over 50 Ohms or an open circuit (infinite resistance) indicates a failed heater element.
• MAF Sensor Reading at Warm Idle — expected: Approximately 2.5 g/s.. Failure: Significantly different readings could indicate a MAF issue causing a rich condition, though this is a rare cause for P0138 alone.

Scan Tool Commands That Help

• Subaru Select Monitor (SSM) or equivalent: Clear Memory Mode — After replacing the sensor, this function should be used to clear the DTC and reset the engine's learned fuel trims. Disconnecting the battery also accomplishes this.
• Subaru Select Monitor (SSM) or equivalent: O2 Heater Diagnosis — The SSM can run a self-test on the O2 heater circuit, which will report as 'Complete' or 'Incomplete'. This can help verify a fix or diagnose a heater circuit fault (P0141) that often accompanies P0138.

Wiring & Ground Locations

• E25 — This is the wiring diagram identifier for the rear oxygen sensor's main harness connector.. Knowing this identifier helps locate the correct connector and pinout on a factory wiring diagram for testing the signal, ground, and heater circuit wires between the sensor and the ECM.
• Rear O2 Sensor Connector — The physical connector is typically clipped to a bracket on the side of the transmission, just above the rear transmission cross-member.. This is the primary point for inspection and testing. It is exposed to the elements and can be a source of corrosion or damage, causing the P0138 code.

OEM Part Supersession History

• 22690AB070 → 22690AB08A, 22690AC091 — The original sensor was prone to internal cracking, leading to short circuits. The new parts have an 'optimized coating' to improve durability as per TSB 09-115-24R.
  Heads up: The Reddit forum citation notes that an aftermarket sensor did not resolve the issue, while the updated OEM part did, highlighting the importance of using a correct, revised part for this specific problem.