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

What's Unique About the 2020-2024 Subaru LEGACY

Subaru is aware of a problem with the original-equipment rear oxygen sensors on this generation of Legacy and many other models. The company issued Technical Service Bulletins (TSBs) acknowledging that the original sensors can fail and cause codes like P0138. 🎬 Watch: Understanding P0138 causes and common circuit fixes. An updated sensor was developed with an "optimized coating application to help prevent cracking within the internal surfaces," which could lead to short circuits. This makes a faulty sensor the most probable cause, more so than on other vehicles where wiring or other issues 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
• Failed emissions test
• Slight decrease in fuel economy
• Rough idling or stalling in some cases
• Strong exhaust odor due to a rich fuel mixture

Most Likely Causes

1. Faulty Rear Oxygen Sensor (Bank 1, Sensor 2) 🔴 High Probability → Shop Oxygen Sensor Subaru issued TSB #09-115-24R and #09-115-24, acknowledging that the original sensors are prone to internal failures that cause a short circuit and high voltage readings. The TSB states the new part has an improved coating to prevent internal cracking.
   How to confirm: Use an OBD-II scanner with live data to monitor the Bank 1 Sensor 2 voltage. If it is stuck high (e.g., above 0.9V) and does not fluctuate, the sensor has likely failed. A healthy downstream sensor on a warm engine should show a relatively stable, slowly fluctuating voltage.
   Typical fix: Replace the rear oxygen sensor with the updated OEM part (22690AB08A) or a high-quality aftermarket equivalent from Denso, the OEM supplier. This is a common DIY job requiring a 22mm (or 7/8") oxygen sensor socket.
   Est. part cost: $100-$250
2. Damaged Wiring or Connector 🟡 Medium Probability The sensor's wiring is located under the vehicle and is exposed to road debris, moisture, and exhaust heat, which can lead to corrosion, chafing, or melting over time. The connector is often located on the frame rail.
   How to confirm: Visually inspect the entire wiring harness from the O2 sensor to its connection point on the frame rail. Check for breaks, melted sections, or corroded pins in the connector. Use a multimeter to check for a short to power on the signal wire.
   Typical fix: Repair or replace the damaged section of the wiring harness or the connector itself.
   Est. part cost: $20-$100

Rare But Worth Checking

• Exhaust System Leak: While more likely to cause other O2 sensor codes, a significant leak before the sensor could potentially alter readings by allowing outside oxygen into the exhaust stream. It's worth a quick inspection while under the vehicle.
• Faulty Engine Control Module (ECM): This is very rare. The ECM should only be considered a potential cause after the sensor and its entire circuit have been thoroughly tested and confirmed to be in good working order.
• Rich Running Condition: Issues like a leaking fuel injector or faulty fuel pressure regulator can cause a genuinely rich exhaust, which the sensor would accurately report as high voltage. However, this is less common than sensor failure on this platform.
• Poor Engine Grounding: Subaru ECUs are known to be sensitive to grounding issues. The main ECU grounds are located on the top of the intake manifold. A poor ground connection between the engine block, intake manifold, and chassis can cause a variety of strange electrical issues, potentially affecting sensor reference voltages. Inspecting the main ground straps for corrosion or looseness is a worthwhile step if the sensor and its direct wiring check out.

Diagnosis Steps

1. Confirm the P0138 code is present using an OBD-II scanner. Check for any other related codes.
2. Using the live data function on the scanner, observe the voltage for 'Bank 1, Sensor 2'. A healthy downstream sensor on a warm engine should show a relatively stable voltage that fluctuates slowly. If the voltage is stuck high (e.g., 0.9V or higher), it confirms the fault.
3. Perform a thorough visual inspection of the rear oxygen sensor's wiring harness and connector. The sensor is located after the catalytic converter, and its connector is typically on the driver's side frame rail. Look for any signs of melting, chafing, corrosion, or loose connections.
4. If the wiring appears intact, the next step is to test the sensor itself. However, given the high failure rate documented by Subaru in TSB #09-115-24R, replacement with the updated part is often the most practical and definitive step.
5. If a new, correct sensor does not resolve the issue, perform advanced circuit testing. This involves checking for a short to power in the signal wire between the ECM and the sensor connector.
6. If the sensor and wiring are confirmed to be good, the final and least likely possibility is a fault within the Engine Control Module (ECM).

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 vehicle, as acknowledged by Subaru's TSBs. The TSB recommends using the updated part for a lasting repair.
  Trusted brands: Subaru (Genuine), Denso (OEM Supplier)
  OEM price range: $120-$250
  Aftermarket price range: $80-$180

Related Codes That Often Appear With This One

• P0137 — This code for 'O2 Sensor Circuit Low Voltage' is also mentioned in the same Subaru TSB and relates to the same sensor failing in a different manner (open circuit).
• P0140 — This code for 'O2 Sensor Circuit No Activity Detected' is another fault mode for the same failing sensor, as noted in TSB #09-115-24R.
• P0141 — This code for the 'O2 Sensor Heater Circuit' is also covered by the same TSB. A failure of the sensor's internal heater can accompany the signal circuit fault.

Technical Service Bulletins (TSBs) & Recalls

• 09-115-24R: Details the availability of new oxygen sensors to address DTCs P0137, P0138, P0140, and P0141 due to potential internal short/open circuits in the original sensors. It applies to 2020-24 Legacy/Outback 2.5L models, among others.
• 09-115-24: The original bulletin, later revised, which first announced the issue and the development of new oxygen sensors to address it.

Platform-Specific Known Issues

• Subaru has issued Technical Service Bulletin #09-115-24R, which supersedes #09-115-24. It announces the availability of new, improved rear oxygen sensors designed to fix recurring P0137, P0138, P0140, and P0141 trouble codes. The bulletin states the new sensors have an 'optimized coating application to help prevent cracking within the internal surfaces'.
• Owner experiences shared on forums confirm that replacing the rear O2 sensor with the updated OEM part (22690AB08A) is the definitive fix. Some owners report the check engine light returning after using an incorrect or non-OEM part, requiring a second replacement with the correct sensor.

Mechanic-Grade Diagnostic Values

• Downstream O2 Sensor (B1S2) Live Data Voltage — expected: A healthy sensor on a warm, running engine should show a relatively stable voltage that fluctuates slowly, typically between 0.5V and 0.8V.. Failure: The voltage is stuck consistently high, above 1.1V or 1.2V.
• O2 Sensor Heater Circuit Resistance — expected: Between 5 and 10 ohms when tested at ambient temperature.. Failure: The resistance is very high (over 50 ohms) or shows an open circuit (infinite resistance). This test confirms the health of the internal heater, which is part of the sensor assembly covered by the TSB.

Hidden / Shadow Codes Worth Checking

• Mode $06, TID:$81 / $82 / $83: These are not shadow codes, but Test IDs within the generic OBD-II Mode $06 function. They report the results of non-continuous monitor tests for the oxygen sensors, such as switching time and response rate. A failing value here can indicate a sensor is degrading before it consistently sets a hard fault like P0138. (see via An advanced OBD-II scan tool that can display Mode $06 data. The tool may or may not translate the manufacturer-specific Test ID (TID) into a human-readable description.)

Scan Tool Commands That Help

• Subaru Select Monitor (SSM4): Current Data Display > O2 Heater Diagnosis — After clearing codes and running the engine, checking this parameter can confirm if the O2 sensor's internal heater circuit test has completed successfully. A status of 'incomplete' or a failure could point to the heater element issue addressed in the TSB, which often accompanies signal circuit faults.
• Subaru Select Monitor (SSM4): Current Data Display > Oxygen sensor output voltage bank one sensor 2 — This is the primary live data parameter to watch. It allows a technician to see the raw voltage output from the sensor in real-time to confirm if it is stuck in a high-voltage state, as the P0138 code suggests.

Wiring & Ground Locations

• Rear O2 Sensor Connector (B19) — The sensor is in the exhaust pipe after the catalytic converter. The wiring harness leads up to a connector, typically mounted on the driver's side frame rail under the vehicle.. This connector is a common point for corrosion or damage. If the sensor is replaced and the code persists, checking for voltage and ground at this connector is the next step. A replacement connector pigtail is available from wiring specialists under identifier B19.
• Main Engine Grounds — Key grounding points for the ECU are located on the top of the intake manifold. The main battery ground cable connects to the chassis near the engine, and additional straps may connect the engine block to the frame.. Subaru engine management is sensitive to poor grounds. A corroded or loose ground on the intake manifold can create a voltage offset, potentially causing the ECU to misinterpret sensor signals. This is an uncommon but possible cause of a P0138 if the sensor and its direct wiring are confirmed good.

Real Owner Repair Stories

• Ultimate Subaru Message Board (Subaru (model unspecified, but behavior is relevant)) — Check Engine Light with O2 sensor heater circuit code (P0135), which is part of the same TSB family as P0138.
  ❌ Tried (didn't work) Replacing the O2 sensor with a generic aftermarket 3-wire sensor using the supplied crimp connectors.
  ✅ What actually fixed it The code kept returning 5-15 minutes into a drive. The user measured the resistance of the new sensor's heater at 5-6 ohms cold, but noted it was higher when warm. The suspected cause, though not definitively resolved in the thread, was that the resistance of the aftermarket sensor and/or the crimped connections was falling out of the ECU's acceptable range once it reached operating temperature. This highlights the importance of using the correct OEM-spec sensor with proper connections, as a small variance in resistance can cause recurring codes.

OEM Part Supersession History

• 22690AB070 → 22690AB08A — The new part has an 'optimized coating application to help prevent cracking within the internal surfaces' that led to short/open circuits in the original design, as per TSB 09-115-24R.
  Heads up: While other sensors may physically fit, using a part other than the updated OEM (or its direct Denso equivalent) has been reported by owners to cause the code to return.