AutoCodes – Subaru – P0151 Subaru

P0151 Subaru Code - HO2S21 Circuit Low Voltage

Quick Answer

The front heated oxygen sensor (or O2 sensor 1) is placed into the exhaust manifold. Common causes include Faulty Front Heated Oxygen Sensor Bank 2, Front Heated Oxygen Sensor Bank 2 harness is open or shorted, Front Heated Oxygen Sensor Bank 2 circuit poor electrical connection. Severity is Moderate - service should be scheduled soon. The vehicle may still be drivable for short trips if it runs normally, but avoid hard driving and repair it soon.

What it means: P0151 Subaru indicates HO2S21 Circuit Low Voltage.
Common causes: Common causes include Faulty Front Heated Oxygen Sensor Bank 2, Front Heated Oxygen Sensor Bank 2 harness is open or shorted, Front Heated Oxygen Sensor Bank 2 circuit poor electrical connection.
Severity: Moderate - service should be scheduled soon.
Can I drive?: The vehicle may still be drivable for short trips if it runs normally, but avoid hard driving and repair it soon.

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Code Popularity 8,010

How popular is this code, among other codes? Low

Repair Importance Level

Moderate (Service soon) (2 / 3)

Repair Difficulty Level

Intermediate (2 / 3)

What is this?

Table of contents Created and updated by William Orellana on Jun 14, 2020

Description
Possible Causes
How To Fix
Tech Notes
Cost to Diagnose
FAQs
Comments
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Code P0151 Subaru Description

The front heated oxygen sensor (or O2 sensor 1) is placed into the exhaust manifold. It detects the amount of oxygen in the exhaust gas compared to the outside air. The heated oxygen sensor 1 has a closed-end tube made of ceramic zirconia. The zirconia generates voltage from approximately 1V in richer conditions to 0V in leaner conditions. The heated oxygen sensor 1 signal is sent to the Engine Control Module (ECM). The ECM adjusts the injection pulse duration to achieve the ideal air-fuel ratio. The ideal air-fuel ratio occurs near the radical change from 1V to 0V.

OBDII Code P0151 Subaru - HO2S21 Circuit Low Voltage - AutoCodes.com — P0151 Subaru Code - HO2S21 Circuit Low Voltage

What does this mean?

What are the Possible Causes of the DTC P0151 Subaru?

Faulty Front Heated Oxygen Sensor Bank 2
Front Heated Oxygen Sensor Bank 2 harness is open or shorted
Front Heated Oxygen Sensor Bank 2 circuit poor electrical connection
Inappropriate fuel pressure
Faulty fuel injectors
Intake air leaks may be faulty
Exhaust gas leaks

How to Fix the DTC P0151 Subaru?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What does this mean?

Code Tech Notes

Replacing the Front Heated Oxygen Sensor Bank 2 usually takes care of the problem.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0151 Subaru code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

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About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P0151 Subaru Code

1. What does OBDII code P0151 mean for a Subaru?

P0151 indicates a low voltage signal from the oxygen sensor (HO2S21) located in Bank 2, Sensor 1, which monitors the air-fuel ratio.

2. What are the common symptoms of code P0151 in a Subaru?

Symptoms may include poor fuel economy, rough idling, reduced engine performance, and an illuminated check engine light.

3. What causes code P0151 in a Subaru?

Common causes include a faulty Bank 2, Sensor 1 oxygen sensor, damaged wiring or connectors, exhaust leaks, or issues with the engine control module (ECM).

4. Can I drive my Subaru with code P0151?

While the vehicle may still be drivable, ignoring P0151 can lead to decreased performance, higher emissions, and potential damage to the catalytic converter.

5. How do you diagnose code P0151 in a Subaru?

Diagnosis typically involves inspecting the oxygen sensor, checking wiring and connectors for damage, and testing the sensorâ€™s voltage with a multimeter.

6. How do you fix code P0151 in a Subaru?

Repairs may include replacing the oxygen sensor, repairing damaged wiring, fixing exhaust leaks, or reprogramming/updating the ECM.

7. How much does it cost to fix code P0151 in a Subaru?

The repair cost varies but typically ranges from $150 to $400, depending on whether the oxygen sensor, wiring, or other components need replacement.

8. Can a dirty air filter cause code P0151 in a Subaru?

While unlikely, a dirty air filter can affect the air-fuel ratio, which might indirectly contribute to sensor issues.

9. Does P0151 always mean the oxygen sensor is bad?

No, the issue could also stem from wiring problems, exhaust leaks, or ECM malfunctions rather than a faulty sensor.

10. How can I prevent code P0151 from recurring in my Subaru?

Regular maintenance, including checking oxygen sensor performance, inspecting wiring, and addressing exhaust leaks promptly, can help prevent recurrence.