Fix P0139: Mechanic’s solutions that WORKED

The code P0139: O2 Sensor Circuit Slow Response Bank 1 Sensor 2 points out a fault in the oxygen sensor’s response time, and most likely is caused by a malfunctioning oxygen sensor or issues in the air and fuel mixture.

P0139 quick overview

Meaning	P0139: O2 Sensor Circuit Slow Response Bank 1 Sensor 2
Is it serious?	Moderate. While the vehicle might still run, it can lead to reduced fuel efficiency, increased emissions, and potential damage to the catalytic converter if not addressed.
Possible causes	– Faulty or aging oxygen sensor – Air leaks in the exhaust system near the O2 sensor – Contaminated or malfunctioning Mass Air Flow (MAF) sensor – Faulty catalytic converters – Issues with the PCV system – Fuel pressure or injector problems
How to diagnose?	Scan for Codes: Always start by scanning the vehicle to confirm the P0139 code and check for any other related codes. Inspect the O2 Sensor: Examine the O2 sensor in question (Bank 1 Sensor 2) for any visible damage or wear. Check for Air Leaks: Inspect the exhaust system, especially near the O2 sensor, for any signs of air leaks. Test the MAF Sensor: If equipped, inspect and test the Mass Air Flow sensor for proper operation. Examine the Catalytic Converters: Check for any signs of damage, rattling, or high back pressure which might indicate a faulty converter. Inspect the PCV System: Especially the valve at the back of the intake manifold, for any malfunction. Check Fuel System: Monitor fuel pressure and inspect injectors for potential leaks or malfunctions.

How to fix P0139 – learn from mechanics

I looked into P0139 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Honda

2005 Honda Accord LX 2.4L

Issue + repair history: MIL lamp on with recurring P0139 code. Rear O2 sensor replaced at 139k miles with a Denso unit. Vehicle returned at 142k with the same code, and the sensor was replaced again with a different Denso part number due to emissions mismatch. Vehicle returned at 144k miles with the same P0139 code. No exhaust leaks, no oil or coolant burning issues, ECM software up to date, no harness/wiring damage, and fuel trims were good. Both front and rear O2 voltages were rapidly changing when a vacuum line was pulled to create a lean condition. The inside of the catalytic converter was inspected and showed no flaking.

Other OBD2 codes + test results: P0139 – Secondary heated O2 sensor slow response (B1S2)

What are people suggesting to fix the issue:

• Verify PCM software and check for TSB regarding the DTC p0139.
• Check for exhaust leaks using a smoke machine.
• Consider replacing with an OEM sensor if the heater circuit is functioning properly.
• Replace the front O2 sensor (A/F ratio sensor).
• Ensure connectors are clean, tight, and no pins are pushed back.
• Quality concerns with brands like Bosch and Denso.
• O2 sensors share power and ground, so one can cause problems with the other.
• Consider OEM parts for the A/F sensor.
• The catalytic converter might have become separated and is too close to or touching the sensor.

What was the final fix: Upon attempting to pull the upstream sensor for inspection, it was found to be hardly snug. It’s suspected that someone replaced the rack after the original rear O2 replacement, possibly affecting the sensor. The issue was potentially related to the upstream sensor’s placement. The vehicle will be monitored to see if it comes back.

Case #2: Hyundai

Vehicle Information 2003 Hyundai Elantra GLS 2.0L

Issue + Repair History Picked up car from used car lot for smog, no check engine light initially. During smog test, misfire monitor was okay, fuel not okay, CCM okay. Check engine light turned on with code po139. An additional 3rd O2 sensor was found added to the back of the catalytic converter and was rewired. Noisy deceleration observed.

Other OBD2 Codes + Test Results po139- rear O2 slow response

What Are People Suggesting to Fix Issue Technician suggests checking for an exhaust leak, especially around the flex-pipe, as they are known for cracking and leaking, which can affect the O2 sensor downstream. Another technician recommends checking for exhaust leaks at the flex pipe, emphasizing that even small leaks can trigger the mentioned code. He also shares a previous experience where cleaning all grounds resolved a similar issue.

What Was the Final Fix Mechanic found an improperly added 3rd O2 sensor at the back of the catalytic converter. The correct rear O2 sensor was installed, and the 3rd one was removed. This action resolved the noisy deceleration issue.

Case #3: Volkswagen

Vehicle Information 2000 Volkswagen Beetle GLS 2.0L

Issue + Repair History VW code p0102 keeps returning as a hard code. Tested MAF, it has power with car running and a ground reading of 2.5k ohms (should be 1.5 ohms). Car runs okay, but there’s suspicion of needing a new MAF. O2 sensors appear to be functioning and are responsive.

Other OBD2 Codes + Test Results
p0102- MAF
p0134 – O2 bank 1 sensor 1
p0139 – O2 bank1 sensor 2

What Are People Suggesting to Fix Issue Technician suggests verifying if there’s a blocked converter and checking if the circuit has 12 volts. Another technician emphasizes using an OE part if replacing the MAF sensor, noting issues with aftermarket sensors. Multiple technicians highlight that MAF problems are common and suggest ordering the MAF from the dealer using the VIN to ensure the correct part. One technician advises checking the scanner readings for the MAF values and recommends running the car with the MAF unplugged to see if performance improves, especially during acceleration under load.

What Was the Final Fix The solution was replacing the MAF with a VW Bosch MAF. After this, the code was cleared, and upon test driving, no SES light appeared.

Case #4: Nissan

Vehicle Information 2002 Nissan Quest SE 3.3L

Issue + Repair History Fault codes P0138 and P0139 related to O2 Sensor Bank 1 Sensor 2. Replaced 226A0-7B503 Oxygen Sensor OE and OE 22680-7B001 Air Mass Sensor. No vacuum leaks detected, and the ignition system is fine. After clearing all faults and driving the car for a few days, the same faults returned after 300 miles. Related repair history includes a new AFM.

Other OBD2 Codes + Test Results P0138- Slow Response bank 1 Sensor 2 P0139- High Voltage bank 1 Sensor 2

What Are People Suggesting to Fix Issue Technician suggests the possibility of a faulty bank 1 sensor 2 or a rare chance of a faulty PCM. Another technician inquires about the front O2 readings at idle and at 2000 rpm, and if adding propane or creating a vacuum leak changes the readings. He also asks about the ground resistance measurement. One technician recommends checking fuel pressure and inspecting injectors for potential leaks into the intake. Another suggestion is to inspect where the front O2 sensor wire meets the alternator wiring harness, as the O2 sensor wires often break at that point. Lastly, a technician asks if the catalytic converters have been checked.

What Was the Final Fix Both O2 sensors and the MAF OE were replaced again for testing purposes, but the same fault returned. The final conclusion was that the PCM was at fault. The customer declined the repair due to the associated cost.

Case #5: BMW

Vehicle Information 1999 BMW 740iL 4.4L

Issue + Repair History Engine pops internally into the intake manifold under hard acceleration. After clearing faults, O2 sensors were checked and found to be okay. Check engine light did not return, but the vehicle has no power under load. In May 2002, the dealer replaced the thermostat map cooling due to fault code 140. Also had fault codes 26 and 27 related to bank 1 and bank 2 multiplicative mixture adaptation.

Other OBD2 Codes + Test Results
po170– fuel trim malfunction bank 1
po173– fuel trim malfunction bank 2
po139- O2 sensor circuit slow response bank 1 & 2

Other tests showed no air leaks and back pressure on both catalytic converters was okay.

What Are People Suggesting to Fix Issue Several technicians suggest checking the Air Mass meter, as it has been a common cause for similar complaints and faults. Some recommend checking the PCV valve at the back of the intake manifold. Others advise inspecting the boot from the airflow meter to the throttle, as they often develop holes at the bottom. A few technicians mention the possibility of broken catalytic converters and recommend checking for rattling or high back pressure. One technician suggests checking the crankshaft sensor and mentions that it might be under warranty.

What Was the Final Fix The issue was resolved by replacing the air mass sensor. The vehicle was taken to the dealer for warranty, as it was under emission warranty.