P0155 Code: Subaru Forester (2019-2024) - Causes, Symptoms & Fixes

No recalls found in NHTSA database.

According to the provided data, there are no NHTSA owner complaints or recalls listed for the Subaru Forester (2019-2024) related to code P0155. This guide uses general OBD-II and Subaru Forester-specific knowledge to help you diagnose and repair P0155, but it is not based on NHTSA incident statistics for this model year range.

1. CODE MEANING AND SEVERITY

• What P0155 means: P0155 is the OBD-II diagnostic trouble code for an O2 Sensor Heater Circuit Malfunction on Bank 2 Sensor 1. In practical terms, the upstream sensor on the second bank (Bank 2) has a heater element that isn’t heating properly, or the heater circuit is not being supplied with proper power/ground.
• Where it sits in the system: Bank 2 Sensor 1 is the upstream O2 sensor on the secondary bank (in a Subaru boxer engine, there are two banks). Heater reliability helps the sensor reach operating temperature quickly for accurate readings.
• Severity and impact: This is typically a non-catastrophic fault. It can cause slower sensor warm-up, higher exhaust emissions until the sensor reaches operating temperature, and a persistent MIL (check engine light). Fuel economy and driveability might be minimally affected if the sensor readings stay reasonable, but the vehicle’s emissions readiness may be impacted and may fail an emissions test until repaired. If accompanied by other O2 sensor or engine codes, symptoms can be more noticeable.

2. COMMON CAUSES ON SUBARU FORESTER

• Faulty Bank 2 Sensor 1 oxygen sensor (heater element failed or sensor degraded).
• Damaged or corroded wiring/connector to Bank 2 Sensor 1 heater circuit (pin, loom, or shield damage; poor connection).
• Blown fuse or failed relay protecting the O2 heater circuit.
• Wiring harness grounding issue or short to power/ground in the sensor circuit.
• Faulty PCM/ECU control for the heater circuit (less common but possible).
• Exhaust or sensor contamination or oil/coolant affecting the sensor or harness.
• Aftermarket wiring alterations or previous repairs that left damaged harnesses.

3. SYMPTOMS

• Check Engine Light (MIL) on with code P0155 stored or pending.
• Longer-than-normal sensor warm-up time after starting the engine, especially in cold weather.
• Possible minor drop in fuel economy or erratic short-term fuel trim behavior (if the sensor reading is slow to come online).
• No obvious performance loss in many cases, but emissions readiness may be incomplete.

4. DIAGNOSTIC STEPS
   Note: This is a logical, step-by-step approach. Use a capable OBD-II scanner and, if needed, a multimeter.

• Step 0: Verify the code

  • Use a scan tool to confirm P0155 is present and check for any related codes (especially other O2 sensor heater codes or sensor performance codes). Note any freeze-frame data (engine load, rpm, temps).

• Step 1: Visual inspection

  • Inspect the Bank 2 Sensor 1 sensor and its wiring harness and connector for obvious damage, chafing, corrosion, oil/coolant contamination, or loose pins.
  • Check for exhaust leaks near the sensor, which can impact readings and sensor life.

• Step 2: Electrical checks

  • Locate the fuse(s) and relay that protect the O2 sensor heater circuit. Confirm the fuse is intact and the relay operates (if applicable). Replace as needed.
  • With the key off, inspect continuity of the heater circuit wires to the sensor. Look for damaged insulation, shorts to ground, or open circuits.

• Step 3: Sensor heater resistance test (engine cold)

  • Disconnect the Bank 2 Sensor 1 electrical connector.
  • Measure the heater element resistance across the heater pins with a digital multimeter (DMM). Typical heater resistance for a heated O2 sensor is a low-ohm value when healthy (often in the single-digit to low tens of ohms range; exact spec varies by sensor and OEM). If the resistance is out of spec or Open/OL, the sensor heater is faulty and the sensor should be replaced.
  • If resistance is within spec, proceed to supply/ground tests.

• Step 4: Supply voltage and ground checks

  • Reconnect the sensor and backprobe or use a current-limited test light to verify:
    • The heater circuit receives 12V (or the vehicle’s system voltage) whenever the ignition is on.
    • The heater circuit has a solid ground path.
  • Look for abnormal voltage drops or intermittent grounding that could cause heater instability.

• Step 5: Commanded heating test (advanced)

  • With a smart scan tool, command the O2 sensor heater on and watch live data for Bank 2 Sensor 1. The heater should draw current and the sensor should reach/maintain expected operating temperature quickly. If the heater does not respond or reads abnormally, the sensor is likely faulty.
  • If the heater circuit shows power and ground but the sensor still reports a fault, replace the sensor.

• Step 6: Rule out contamination or installation issues

  • If oil, fuel, or coolant leaks have contaminated the sensor, address the leak first and consider sensor replacement after cleaning the area.
  • If the sensor was recently replaced and the code persists, re-check the wiring and connector orientation and ensure the proper sensor type was installed (correct bank and sensor 1 vs sensor 2).

• Step 7: Clear codes and validate

  • After repairs, clear the codes and drive the vehicle through a complete drive cycle to verify the code does not return and the readiness monitors set as expected.

5. RELATED CODES

• P0135 or other O2 sensor heater-related codes on the Subaru Forester family (general context for heater circuit diagnosis across multiple sensors/banks).
• Other oxygen sensor codes (e.g., P0130-P0134 for Bank 1 sensors, P0150-P0154 for Bank 2 sensors) may appear if multiple heater circuits are affected or if there are concurrent sensor issues.
• P0141 or P0146 (O2 sensor heater circuit issues for downstream sensors) may be seen in conjunction with upstream heater problems in some scenarios.
  Note: The exact related codes depend on which sensors are reporting and the vehicle’s wiring/ECU logic.

6. REPAIR OPTIONS AND COSTS (2025 PRICES)
   Prices vary by region, shop, and whether OEM or aftermarket sensors are used. The following are approximate ranges to help with budgeting.

• Oxygen sensor (Bank 2 Sensor 1) replacement parts
  • Aftermarket sensor: roughly $50–$150
  • OEM/subaru oem equivalent: roughly $100–$250
• Labor (professional shop)
  • Typical bolt-in sensor replacement takes about 0.5–1.5 hours depending on accessibility; labor cost commonly ranges from $80–$150 per hour. Total labor usually around $40–$225 (often around $100–$200 in many markets).
• Total estimated repair cost (parts + labor)
  • DIY: Part cost only ≈ $50–$250 (plus tools)
  • Professional shop: Approximately $150–$450 total (depending on sensor choice and labor rates)
• Additional costs to consider
  • Wiring harness repair or connector replacement: $40–$200 extra if damage found
  • Exhaust leak repair if an off-sensor leak affects readings: $100–$350
  • ECU/PCM replacement is rarely needed; if required, costs rise significantly (several hundred to over $1,000 in extreme cases)

7. DIY VS PROFESSIONAL

• DIY pros
  • Typical O2 sensor replacement is a common DIY task with basic hand tools and an O2 sensor socket.
  • Pros: Lower cost, immediate turnaround, can do it with the car on the ground.
  • Cons: Risk of cross-threading, damaging harness, or missing other related issues; sensors can be stubborn to remove if corroded; ensure proper torque on the new sensor.
• DIY cons
  • If wiring harness damage or ECU control is suspected, a professional diagnostic is advised.
  • Always disconnect the battery when working on sensor circuits to avoid shorts.
• Professional path
  • Recommended if you suspect wiring/connector problems, need precise heater circuit testing, or if you want a formal diagnostic report for emissions testing.
  • A shop can perform confirmed heater resistance tests, power/ground integrity checks, and ensure proper catalytic converter readiness.

8. PREVENTION

• Regularly monitor OBD-II codes and emission readiness. Address codes early to prevent catalyst or sensor damage.
• Use high-quality oxygen sensors with proper ratings for Subaru engines; avoid low-cost aftermarket parts that may fail prematurely.
• Address exhaust leaks promptly, as leaks can sabotage sensor readings and heater operation.
• Fix engine oil leaks or coolant leaks promptly to prevent sensor contamination.
• Replace O2 sensors according to the vehicle’s recommended service intervals or earlier if reading drift or heater symptoms appear.
• Inspect wiring harnesses and connectors for wear, rubbing against exhaust components, or corrosion; secure harnesses away from heat sources.

Important data note

• No recalls found in NHTSA database for this issue on the 2019-2024 Subaru Forester according to the provided data.
• No NHTSA complaints found for this make/model/year/issue combination, per the provided data. This guide relies on general automotive knowledge and standard OBD-II practice in addition to the specific P0155 definition.