P0138 on 2020-2024 Subaru Outback: O2 Sensor High Voltage Causes and Fixes

What's Unique About the 2020-2024 Subaru OUTBACK

For this generation of Subaru Outback, the P0138 code is a known, widespread issue. Subaru released Technical Service Bulletins (TSBs #09-115-24 and #09-115-24R) acknowledging that the original rear oxygen sensors were prone to failure. The TSB explains that the failure is due to internal cracking, which leads to a short circuit and the high voltage reading. The bulletins announce the availability of a redesigned sensor with an "optimized coating application" to prevent this cracking. This makes a faulty sensor the most probable cause by a wide margin, more so than on other vehicles where wiring or other factors might be more common.

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
• Vehicle will fail an emissions test
• Rough or unstable idle
• Potential for a strong exhaust odor due to a false rich reading

Most Likely Causes

1. Faulty Rear Oxygen Sensor (Bank 1, Sensor 2) 🔴 High Probability → Shop Oxygen Sensor Subaru issued TSBs #09-115-24 and #09-115-24R regarding a known defect. The TSB states the original sensors can develop internal cracks, causing a short circuit and a high voltage reading. An updated part was released to fix this. Forum users on subaruoutback.org report this failure is common between 30,000 and 60,000 miles.
   How to confirm: Use an OBD-II scanner to monitor the live data for the B1S2 O2 sensor. If the voltage is stuck high (e.g., >0.9V, often at 1.2V) and does not fluctuate, the sensor has failed as described in the TSB. A healthy downstream sensor should show a slow, steady voltage, typically around 0.5-0.7V.
   Typical fix: Replace the rear oxygen sensor (Bank 1, Sensor 2) with the updated OEM part, number 22690AB08A. A high-quality aftermarket equivalent from Denso (the OEM supplier) is also a reliable option.
   Est. part cost: $120-$250
2. Damaged Wiring or Connector 🟡 Medium Probability The sensor and its wiring are located underneath the vehicle, exposed to road debris, moisture, and exhaust heat, which can lead to corrosion, frayed wires, or a short to power. The connector is often located on the frame rail.
   How to confirm: Visually inspect the wiring harness leading from the rear O2 sensor to its connector on the frame rail. Look for any signs of physical damage, melting, or corrosion at the connector pins. A multimeter can be used to check for a short to power in the signal wire.
   Typical fix: Repair the damaged section of the wiring harness or clean/replace the corroded connector.
   Est. part cost: $10-$100

Rare But Worth Checking

• Exhaust Leak: An exhaust leak before or near the rear O2 sensor can introduce outside air, which can sometimes cause unusual sensor readings. While more commonly associated with low voltage codes, it's worth a quick inspection.
• Engine Control Module (ECM) Fault: In very rare cases, the ECM itself can be the source of the problem, either by misinterpreting the sensor's signal or having an internal fault. This should only be considered after all other possibilities have been ruled out.

Diagnosis Steps

1. Connect an OBD-II scanner to confirm the P0138 code and check for any other related codes.
2. Use the scanner's live data function to observe the voltage from 'O2S B1S2' (Bank 1, Sensor 2). Confirm if the voltage is stuck high, typically above 0.9V and often at or near 1.2V.
3. Perform a thorough visual inspection of the rear oxygen sensor's wiring harness and its connector on the frame rail. Look for any signs of melting, chafing, corrosion, or loose connections.
4. If the wiring appears intact, the most likely cause is a failed sensor, as documented in Subaru's TSB 09-115-24R.
5. If you have access to a multimeter, you can disconnect the sensor and test the wiring harness for a short to power on the signal wire.
6. Test the sensor's internal heater circuit resistance with a multimeter. A typical reading for a good sensor is between 3 and 10 ohms. An open circuit (infinite resistance) or a short (near zero ohms) indicates a failed sensor.
7. If the wiring and connector are confirmed to be good, replace the rear oxygen sensor. A specialized 22mm O2 sensor socket is recommended for removal.
8. After replacement, clear the trouble codes with the scanner and perform a drive cycle to ensure the code does not return.

Parts You'll Likely Need

• Rear Oxygen Sensor (Bank 1, Sensor 2) (OEM #22690AB08A) — This is the most common failure point for this code on this specific vehicle, as acknowledged by Subaru's TSB 09-115-24R. This is the updated part number designed to prevent the issue from recurring.
  Trusted brands: Subaru (OEM), Denso, NGK/NTK
  OEM price range: $150-$250
  Aftermarket price range: $80-$160

Related Codes That Often Appear With This One

• P0137 — This code for 'O2 Sensor Circuit Low Voltage' can appear with P0138 if the sensor is failing erratically or there is an intermittent wiring problem. Both are mentioned in the same Subaru TSB.
• P0140 — 'O2 Sensor Circuit No Activity Detected' can also be triggered by a completely failed sensor or wiring issue, and is also part of the same Subaru TSB.
• P0141 — This code for 'O2 Sensor Heater Circuit' failure is common to see alongside sensor voltage codes, as a faulty heater can cause the sensor to send incorrect voltage readings. It is also covered by the same TSB.

Technical Service Bulletins (TSBs) & Recalls

• 09-115-24R: Addresses DTCs P0137, P0138, P0140, and P0141 and the availability of new, improved oxygen sensors with an optimized internal coating.
• 09-115-24: The original bulletin announcing the development of new oxygen sensors to address these specific DTCs.

Platform-Specific Known Issues

• Subaru has issued Technical Service Bulletins #09-115-24 and #09-115-24R regarding this code. The bulletins announce the availability of a new, improved rear oxygen sensor designed to prevent the failures that cause P0138 and related codes.
• The TSB states the cause is an internal crack in the sensor element, and the new part has an 'optimized coating' to prevent this.
• Owner forums confirm this is a very common failure, often occurring before 60,000 miles, and that replacing the sensor with the updated part number is the correct fix.

Mechanic-Grade Diagnostic Values

• Rear O2 Sensor (B1S2) Heater Circuit Resistance — expected: 5 to 10 Ohms. Failure: A reading of infinity (O.L.) indicates an open circuit, while a reading near zero indicates a short. Either confirms a bad sensor.
• Rear O2 Sensor Installation Torque — expected: 40 Nm (30 ft-lbs). Failure: Over-tightening can damage the new sensor, while under-tightening can cause exhaust leaks.

Scan Tool Commands That Help

• Subaru Select Monitor (SSM) or generic OBD-II scanner: Clear Memory / Clear DTCs — After replacing the sensor, the codes must be cleared from the ECM's memory. The computer will then relearn fuel trims with the new, functioning sensor.

Wiring & Ground Locations

• E25 / B19 / B95 — These are wiring diagram IDs for the rear oxygen sensor connector plug, which is typically located on a bracket on the frame rail or transmission bellhousing.. Knowing the connector ID helps locate the correct plug for testing the harness side for power, ground, and signal integrity back to the ECM, ruling out a wiring issue.

OEM Part Supersession History

• 22690AB070 → 22690AB08A — The original sensor was prone to internal cracking, causing a short circuit. The new part has an improved design to prevent this failure, as noted in TSB 09-115-24R.