AutoCodes – Mazda – P0138 Mazda

P0138 Mazda Code - HO2S RR Circuit High Input

Quick Answer

The Heated Oxygen Sensor 2 (HO2S) (Rear O2 Sensor), after three way catalyst (manifold), monitors the oxygen level in the exhaust gas on each bank. Common causes include Faulty Rear Heated Oxygen Sensor Bank 1, Rear Heated Oxygen Sensor Bank 1 harness is open or shorted, Rear Heated Oxygen Sensor Bank 1 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: P0138 Mazda indicates HO2S RR Circuit High Input.
Common causes: Common causes include Faulty Rear Heated Oxygen Sensor Bank 1, Rear Heated Oxygen Sensor Bank 1 harness is open or shorted, Rear Heated Oxygen Sensor Bank 1 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.

Save to My Repairs Step-by-Step Diagnostic Guide My Repairs

AI answers can be wrong. Verify repairs with testing. Open full chat

Code Popularity 43,154

How popular is this code, among other codes? Medium

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
Step-by-Step Diagnostic Guide
Possible Causes
How To Fix
Tech Notes
Cost to Diagnose
FAQs
Comments
Add Comment

💬 Need more help? Ask Coddy for instant answers — it's free!

Code P0138 Mazda Description

The Heated Oxygen Sensor 2 (HO2S) (Rear O2 Sensor), after three way catalyst (manifold), monitors the oxygen level in the exhaust gas on each bank. For optimum catalyst operation, the air fuel mixture (air-fuel ratio) must be maintained near the ideal stoichiometric ratio. The HO2S output voltage changes suddenly in the vicinity of the stoichiometric ratio. The Engine Control Module (ECM) adjusts the fuel injection time so that the air-fuel ratio is nearly stoichiometric. The HO2S generates a voltage between 0.1 and 0.9 V in response to oxygen in the exhaust gas. If the oxygen in the exhaust gas increases, the air-fuel ratio becomes Lean. The ECM interprets Lean when the HO2S voltage is below 0.45 V. If the oxygen in the exhaust gas decreases, the air-fuel ratio becomes Rich. The ECM interprets Rich when the HO2S voltage is above 0.45 V.

OBDII Code P0138 Mazda - HO2S RR Circuit High Input - AutoCodes.com — P0138 Mazda Code - HO2S RR Circuit High Input

What does this mean?

What are the Possible Causes of the DTC P0138 Mazda?

Faulty Rear Heated Oxygen Sensor Bank 1
Rear Heated Oxygen Sensor Bank 1 harness is open or shorted
Rear Heated Oxygen Sensor Bank 1 circuit poor electrical connection
Intake air leaks
Exhaust gas leaks
Inappropriate fuel pressure
Faulty fuel injectors

How to Fix the DTC P0138 Mazda?

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 Rear Heated Oxygen Sensor Bank 1 usually takes care of the problem.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0138 Mazda 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.

Need more information about the P0138 Mazda code?

Need more information on how to fix the P0138 Mazda code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P0138 Mazda code diagnosed by a professional: Find a repair shop in your area

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 P0138 Mazda Code

1. What are the common symptoms of OBDII code P0138 in a Mazda?

- Common symptoms include poor fuel economy, rough idling, and an illuminated check engine light.

2. What does the HO2S RR Circuit High Input mean in relation to code P0138?

- It indicates that the rear oxygen sensor in bank 1 is detecting a high voltage input, which usually points to an issue with the oxygen sensor or its circuit.

3. What are the possible causes of OBDII code P0138 in a Mazda?

- Potential causes include a faulty rear oxygen sensor, wiring issues in the sensor circuit, exhaust leaks, or a problem with the engine's air-fuel mixture.

4. How can I diagnose the HO2S RR Circuit High Input issue in my Mazda?

- Diagnosing this issue usually involves using a scan tool to read the code, inspecting the rear oxygen sensor and its wiring, and checking for any exhaust leaks.

5. Can I still drive my Mazda with code P0138 present?

- It's generally not recommended to drive with this code present for an extended period as it can lead to decreased fuel efficiency and potential damage to the catalytic converter.

6. How can I repair OBDII code P0138 in my Mazda?

- Repair options include replacing the rear oxygen sensor, fixing any wiring issues, addressing exhaust leaks, and ensuring the engine's air-fuel mixture is within specifications.

7. Will resetting the check engine light make OBDII code P0138 go away permanently?

- Resetting the check engine light may temporarily turn it off, but if the underlying issue causing the code is not fixed, the light will likely come back on.

8. Is it possible to prevent OBDII code P0138 from occurring in my Mazda?

- Regular maintenance, such as ensuring proper engine tune-ups, checking for exhaust leaks, and replacing oxygen sensors as recommended, can help prevent this code from occurring.

9. How much does it typically cost to repair OBDII code P0138 in a Mazda?

- The cost can vary depending on the specific cause of the issue, but it may range from around $100 to $300 for parts and labor.

10. Can code P0138 be caused by a faulty catalytic converter in a Mazda?

- While a faulty catalytic converter is not a common cause of P0138, it can indirectly contribute to sensor malfunctions due to improper exhaust gas processing.