P0155 Code: Subaru Outback (2020-2024) - Causes, Symptoms & Fixes

No recalls found in NHTSA database

No NHTSA complaints found for this make/model/year/issue combination.

Important data note: The information below combines general OBD-II knowledge for P0155 with practical Subaru Outback-specific considerations. The provided data indicates no official NHTSA recalls or owner complaints for this exact issue and model range in the supplied dataset, so you won’t find model-year specific recall guidance here. Use this guide as a diagnostic and self-check framework, and verify with your dealer or a qualified shop if you suspect anything outside normal operation.

CODE MEANING AND SEVERITY

• Code meaning: P0155 = O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1). This indicates the heater element in the upstream oxygen sensor on Bank 2 is not heating as intended. The heater helps the sensor reach its optimal operating temperature more quickly, reducing warm-up time and improving emission control readiness.
• Bank terminology: On Subaru Outback (2020–2024), Bank 2 Sensor 1 refers to the upstream O2 sensor for the second exhaust bank. In practical terms, it’s the heated oxygen sensor located before the catalyst on the “other” bank relative to Bank 1.
• Severity and impact: Moderate. A heater problem often causes slower sensor warm-up, longer time for the PCM to get accurate readings, and possible degraded emissions performance. The MIL (check engine light) may be on, and fuel economy and drivability can be affected, especially under cold starts or during emissions testing. In some cases, the vehicle may still run in open-loop for a period if the sensor isn’t heating properly.
• Likelihood of other issues: The heater circuit fault is frequently due to a defective sensor, damaged wiring, or a blown fuse/poor connection. Less commonly, it can be a PCM/ECU fault. If other O2 sensor or exhaust problems exist (vacuum leaks, misfires, fuel delivery issues), they can compound the symptoms.

COMMON CAUSES ON SUBARU OUTBACK

• Faulty Bank 2 Sensor 1 O2 sensor (internal heater element failed).
• Damaged or corroded wiring harness or connector for Bank 2 Sensor 1 (chafing, heat damage, water intrusion, poor pin connection).
• Blown fuse or relay protecting the O2 heater circuit (check fuse box and any relevant relays).
• Poor or missing ground for the heater circuit or sensor (ground path can affect heater operation).
• Exhaust heat shield contact or sensor mounting causing vibration or heat-related failure.
• Sensor contamination or carbon buildup on the sensor tip (affecting sensor operation and readings; may coincide with heater issues).
• Age-related wear: O2 sensors are wear items; extended exposure to heat and exhaust contaminants reduces heater effectiveness over time.
• Less common: PCM/ECU fault or software issue affecting heater command (rare, but possible in some platforms).

SYMPTOMS

• Check Engine Light (MIL) on or intermittent.
• Reduced or inconsistent fuel economy.
• Cold-start or startup drive cycles with slower sensor warm-up; longer in open-loop fuel maps.
• Potentially noticeable emissions test failure indications.
• Occasional rough idle or changes in engine behavior if multiple O2-related codes appear in tandem.
• No obvious noisy symptoms; exhaust leaks around the sensor area can manifest as a change in idle or exhaust note but are less typical as a primary P0155 cause.

DIAGNOSTIC STEPS

Note: When working with O2 sensors and exhaust components, perform diagnostics with the engine at a safe operating temperature and follow all shop manual safety procedures.

• Step 0: Confirm the code and conditions

  • Use an OBD-II scanner to confirm P0155 is present and note any freeze-frame data.
  • Check for other stored codes (especially other O2 sensor codes) that could indicate a broader sensor or fuel system issue.
  • Confirm the MIL is on or off and whether readiness monitors are affected.

• Step 1: Visual inspection

  • Inspect Bank 2 Sensor 1 wiring harness and connector for signs of damage, corrosion, or loose pins.
  • Look for heat damage, chafing against the exhaust, or water intrusion at the connector.
  • Check for exhaust leaks near the sensor that could alter readings or wiring integrity.

• Step 2: Electrical checks on the heater circuit

  • Locate Bank 2 Sensor 1 and identify its heater circuit wires (heater supply and ground).
  • Check for power supply to the heater circuit with the key on (and engine off) per service manual, and measure ground integrity.
  • Use a multimeter to check the sensor’s heater resistance if the sensor is removed (typical heater resistance is a single low-ohm path; consult the vehicle’s service data for the exact spec).
  • With the sensor connected, monitor live data on a scan tool. Observe the heater current/readout as the engine warms up. If the heater current does not rise or if the heater data remains inoperative, the heater circuit or sensor is suspect.
  • Check for blown fuses specific to the O2 heater circuit and any relevant wiring harness protection.

• Step 3: Sensor swap test (if practical)

  • If you have a known-good Bank 2 Sensor 1 sensor available, swap it temporarily (or swap with Bank 1 Sensor 1 if you can) to see if the code clears or moves. If the code clears with the swapped sensor, the original sensor is faulty.

• Step 4: Inspect and test for external causes

  • Vacuum leaks, MAF issues, or fuel delivery irregularities can influence O2 sensor readings; address any obvious issues.
  • Ensure engine oil or coolant deposits aren’t contaminating the sensor area (oil/well-used sensors can degrade heater performance).

• Step 5: Decide on repair path

  • If the heater circuit is confirmed open/short or the sensor heater current is not present despite a good connector and fuse, replace Bank 2 Sensor 1 O2 sensor.
  • If wiring harness damage is found, repair/replace the harness or connectors as needed.
  • If the sensor tests good and wiring checks out, consider PCM fault or software-related command issues as a last resort (rare).

• Step 6: Post-repair validation

  • After repair or replacement, clear codes and perform a road test to verify the code does not return.
  • Monitor live data through the warm-up period and ensure the O2 sensor heater is drawing current and reaching operating temperature promptly.
  • Confirm all readiness monitors pass after testing.

RELATED CODES

• Other O2 sensor heater circuit fault codes (for Bank 1 and Bank 2 sensors, upstream and downstream) that you might encounter in the same family include general O2 sensor heater-related or circuit fault codes during diagnostics. Be prepared to see concurrent or sequential codes if multiple sensors or circuits are affected.
• Also monitor for generic O2 sensor circuit fault codes (e.g., sensor not switching or slow response) that can accompany or follow a heater issue.
• In practice, if you find P0155, you may also encounter related O2 heater or circuit codes for the other bank’s sensors, or other O2 sensor circuit faults, depending on the illness of the wiring or sensors in the system.

REPAIR OPTIONS AND COSTS (2025 PRICES)

Notes:

• Prices vary by region, shop, parts supplier, and whether OEM or aftermarket sensors are used.

• Labor times are estimates; actual times depend on shop labor rates and vehicle access challenges.

• Bank 2 Sensor 1 O2 sensor replacement

  • Parts: $50–$180 (aftermarket) or $100–$250 (OEM).
  • Labor: ~0.5–1.5 hours depending on access and whether exhaust components must be moved.
  • Total ballpark: $180–$400 (common range; higher if additional labor is needed).

• Wiring harness or connector repair

  • Parts: $20–$120 (connector, wires, seals) plus any incidental harness sections.
  • Labor: 0.5–2.0 hours depending on damage extent.
  • Total ballpark: $150–$600+.

• fuse/relay replacement (if the issue is a blown fuse or faulty relay)

  • Parts: $5–$20.
  • Labor: minimal if done by owner or quick shop repair.
  • Total ballpark: $15–$60.

• PCM/ECU considerations (rare)

  • Parts: $300–$800.
  • Labor: 1–3 hours or more if programming or immobilizer procedures are involved.
  • Total ballpark: $450–$1,400.

• Note on catalytic converter or major exhaust work

  • If long-term issues cause catalyst damage or if exhaust components require replacement to restore proper sensor operation, costs can rise significantly.

DIY VS PROFESSIONAL

• DIY-friendly tasks:

  • Visual inspection of wiring and connectors.
  • Basic electrical checks with a multimeter (power, ground, and continuity checks on the heater circuit, if you’re comfortable).
  • Replacing the Bank 2 Sensor 1 O2 sensor using standard anti-seize guidance on threads (do not apply anti-seize to the sensor tip). Use an O2 sensor socket, be mindful of heat and exhaust location, and follow the sensor manufacturer’s torque recommendations.
  • Clearing codes and monitoring live data with a scan tool after repair.

• Tasks that benefit from a professional:

  • Diagnosing intermittent heater faults that require more advanced wiring trace techniques or use of specialized scan tools.
  • Confirming heater current with proper equipment and verifying fuse and relay operation under load.
  • Replacing exhaust components or accessing awkward sensor mounting locations.
  • Any suspected PCM/ECU fault or software updates, which should be performed by a dealer or qualified shop.

• Safety considerations:

  • Exhaust components and catalytic areas get extremely hot; work with engine cold and follow proper hot-work precautions.
  • Disconnect the battery before unplugging sensor connectors to avoid short circuits.
  • Use proper anti-seize on sensor threads as recommended by the sensor manufacturer (avoid getting anti-seize on the sensor tip or wiring).

PREVENTION

• Regular maintenance and inspection

  • Inspect O2 sensor wiring and connectors during routine maintenance, especially if you’ve had exhaust work, heat shields removed, or harsh driving conditions.
  • Address any exhaust leaks promptly, as leaks around sensor locations can cause erroneous readings or sensor damage.

• Use quality parts

  • When replacing sensors, use reputable OEM or high-quality aftermarket sensors designed for Subaru Outback applications.
  • Ensure any replacement wiring harness or connectors are rated for automotive environment and heat exposure.

• Monitor driving conditions

  • If you frequently drive in extreme temperatures or short trips where the engine never fully warms, sensors may experience more wear due to repeated temperature cycling. Occasional longer trips can help sensors reach and maintain operating temperature.

• Observe proper installation practices

  • Ensure sensors are torqued to spec and seated correctly.
  • Avoid overtightening or cross-threading the sensor.
  • Keep sensors and connectors clean and dry; seal connectors properly to prevent corrosion.

• Emissions testing readiness

  • If you’re approaching an emissions test, ensure the ECU has completed readiness monitors after any repair and that the MIL is off prior to testing.

Final notes on data and applicability

• The provided dataset indicates no NHTSA complaints for this exact issue on 2020–2024 Subaru Outback and no recalls in the database. This guide uses general OBD-II principles and Subaru-typical layouts to help you diagnose P0155 (Bank 2 Sensor 1 O2 heater circuit fault) in this vehicle family. Real-world results may vary by model year, engine variant (e.g., 2.5L vs. 2.4L turbo), and individual vehicle history.