Fix P0136: Mechanic’s solutions that WORKED

The code P0136: Rear O2 Sensor Circuit Malfunction points out a fault in the oxygen sensor located after the catalytic converter (often referred to as the post-cat sensor or downstream sensor). This sensor plays a crucial role in monitoring the efficiency of the catalytic converter and ensuring optimal fuel economy. The most common causes for this code are issues with the O2 sensor itself, wiring problems, or a malfunctioning catalytic converter.

P0136 quick overview

Meaning	P0136: P0136: Rear O2 Sensor Circuit Malfunction (Bank 1 sensor 2)
Is it serious?	Yes, it can affect fuel efficiency and emissions. If left unchecked, it can lead to more significant issues, especially with the catalytic converter.
Possible causes	– Faulty or failing rear O2 sensor – Wiring or connection issues related to the O2 sensor – Malfunctioning catalytic converter – Issues with the vehicle’s computer or ECM – Exhaust leaks
How to diagnose?	Scan the Vehicle: Use a diagnostic tool to confirm the presence of the P0136 code and check for any other related codes. Inspect the O2 Sensor: Physically check the rear O2 sensor for any visible damage or wear. Check Wiring and Connectors: Examine the wiring and connectors leading to the O2 sensor for any signs of damage, corrosion, or disconnection. Monitor O2 Sensor Activity: Using a scanner or oscilloscope, monitor the activity of the rear O2 sensor to see if it’s functioning correctly. Inspect the Catalytic Converter: Check the catalytic converter for signs of malfunction or damage. Check for Exhaust Leaks: Inspect the exhaust system for any leaks, especially near the O2 sensor and catalytic converter. Reset and Test Drive: After making any repairs, clear the code and take the vehicle for a test drive to see if the code reappears. Consider Other Factors: If the issue persists, consider factors like the need to reset adaptations, the quality of the replacement parts used, or potential issues with the vehicle’s computer.
What is Bank1 sensor 2?	Post-cat oxygen sensor for engine head 1. Read the full Bank1 vs Bank2 explanation.

How to fix P0136 – learn from mechanics

I looked into P0136 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: Ford

1996 Ford F-150 Eddie Bauer 4.9L

Issue + repair history:

• MIL code: P0136 H02S12 circuit problem.
• Truck initially had codes P0172 and P0173 indicating lean conditions on both left and right with LFT + 25%.
• Low fuel pressure was detected.
• A new fuel pump was installed which improved the truck’s performance.
• However, a problem with the post-catalyst O2 sensor emerged after the truck ran an OBDII self-diagnostic.
  

What are people suggesting to fix the issue:

• Check the downstream O2 sensor heater circuit. Specifically, check continuity from the PCM to the sensor connector and the resistance across the heater in the O2 sensor. If there’s an open circuit, replace the sensor. If everything checks out, a faulty PCM might be the culprit.
• Monitor the rear O2 sensor data to see if it’s cycling when hot or if the signal goes flat. The catalytic converter might be worn out from the lean condition or the rear O2 sensor might be malfunctioning.
• Check the long fuel trims, barometric pressure, mass air flow voltage, and see if the vehicle is missing.
• If the vehicle has an AIR pump, ensure it’s functioning properly as it can cause the catalytic converter to fail the monitor test.
• Consider replacing both O2 sensors, especially if the vehicle has high mileage.
  

What was the final fix:

• The technician replaced the O2 sensor which allowed the computer to continue with efficiency tests.
• Unfortunately, the catalyst failed the test, resulting in a P0420 code indicating poor catalytic efficiency on bank 1.
• The lean running condition over time likely damaged both the post-catalytic sensor and the converter.
• The truck, having 150k miles on it, might have been due for a replacement anyway.
• The technician now needs to recommend a catalytic converter replacement to the owner.

Case #2: Toyota

2001 Toyota Sienna LE 3.0L

Issue + repair history:

• Check Engine Light On.
• Initial scan showed code P0136.
• Both O2 sensors were replaced.
• Check Engine Light returned the next day with the same code.
• Rear O2 reading was 0.00V and 2 of the front sensors were not ready.
• Both Air Fuel Ratio Sensors were replaced with Toyota parts.
  

What are people suggesting to fix the issue:

• Check the long-term fuel trims and see if the O2 sensors respond when the mixture is artificially enriched.
• P0136 is for Bank 1, Sensor 2. The front ones are A/F sensors and the rear ones are regular O2 sensors.
• Check the intake air sensor and consider cleaning it.
• Ensure the correct sensors were replaced; P0136 refers to the O2 sensor after the catalytic converter.
• Consider cleaning the MAF sensor.
• Check for exhaust leaks in front of the downstream sensor.
• Inspect the wiring for any issues, especially a shorted signal wire from the ECM.
  

What was the final fix:

• The vehicle had two air fuel ratio sensors and one O2 sensor.
• The O2 sensor was replaced.
• After a long drive, the Check Engine Light did not return.
• It was noted that running lean for a period might have damaged the post-catalytic sensor and the converter.

Case #3: Nissan

1995 Nissan Maxima 3.0L

Issue + repair history:

• MIL (Check Engine Light) On.
• Poor fuel economy reported.
• This is the second ’95 Maxima with a PO136 code (Rear O2 sensor) in a short time span.
• The previous Maxima had its rear O2 sensor replaced but returned with the same PO136 code.
• The rear sensor’s voltage readings remained consistent even after replacement.
• Readings were taken from an OBDII scanner at the connector near the ECU.
  

What are people suggesting to fix the issue:

• The rear O2 sensor’s malfunction might not directly cause poor fuel mileage.
• Doubts about whether the rear sensor could influence fuel economy.
• The consistent voltage readings suggest it might not be a circuit problem.
  

What was the final fix:

• The rear O2 sensor was replaced again, this time with a factory part.
• The problem did not return after using the factory part.
• Consideration to only use O2 sensors from the dealer in the future.

Case #4: Hyundai

1997 Hyundai Accent GL 1.5L

Issue + repair history:

• MIL (Check Engine Light) On.
• Vehicle failed the emissions test.
• No driveability problems; emissions are extremely low.
• Initial code was p0133 (b1s1 slow response). After replacing the front O2 sensor, the code changed to p0136 (b1s2 malfunction).
• Discrepancy between the actual voltage readings on the O2 sensors and the readings from the OTC scan tool.
  

What are people suggesting to fix the issue:

• Check if the vehicle uses a floating O2 ground.
• Check the vehicle’s grounds, especially if there’s a voltage difference between the scope and the scan tool.
• After replacing the O2 sensor, consider reprogramming the ECM.
• Measure O2 sensor signals and voltage directly at the ECM.
  

What was the final fix:

• Replaced the rear O2 sensor with a new OEM part.
• Rear O2 voltage began to cycle between 1.0v and 1.4v.
• Code p0136 did not return, and the vehicle passed the emissions test.
• There remains a discrepancy between scan data and actual voltage, which will be further investigated on other Hyundai models.

Case #5: Volvo

1999 Volvo V70 2.4L

Vehicle Information:

• 1999 Volvo V70, 2.4L Engine
  

Issue + repair history:

• MIL (Check Engine Light) On.
• Code for rear O2 sensor (P0136).
• Replaced rear O2 sensor with OE Bosch, but the light came back on in 5 minutes.
• Previously had codes for the front sensor and replaced the front sensor a few months prior.
  

What are people suggesting to fix the issue:

• Check the O2 activity; if the rear O2 switches similarly to the front O2, there might be a catalytic converter problem.
• Use OE sensors and consider getting a special harness adapter kit.
• Disconnect power to the ECU to clear the adaptation after replacing oxygen sensors.
• Check for an oily residue in the connectors; if present, clean them out and get a pigtail adapter harness when replacing the sensors again.
• After clearing codes, drive a minimum of 2 test drives to clear them from memory and turn off the light.
• Reset adaptive memory (disconnect the battery) to get the P0136 code to go away.
  

What was the final fix:

• Disconnected the battery power to reset adaptations for the O2 sensor, which cleared the code.
• After driving, the rear O2 sensor was working properly, but the voltage was a bit high, suggesting that a cat efficiency code might appear soon.