The onboard diagnostic (OBD) code P0136 is defined as an Oxygen Sensor Circuit Low Voltage in Bank 1, Sensor 2. It refers to the second oxygen sensor in Bank 1 of the engine, which is on the side of the engine with the #1 cylinder. Typically, the oxygen sensor produces a voltage between 0.1 mV (millivolts) and 0.9 mV.
The ECM (Engine Control Module) monitors the oxygen sensor’s voltage and determines if the exhaust is running lean or rich. It is the ECM’s job to increase or decrease fuel injection input according to the engine’s fuel/air ratio, reported by the oxygen sensor. When the exhaust is running lean (not enough oxygen), the oxygen sensor’s voltage will be too low. If the ECM detects that the voltage is too low for an extended period of time (the length of time varies depending on the make and model of the vehicle), it will set the P0136 code.
- Lit check engine light
Sometimes the driver will not notice any other symptoms related to the P0136 code besides the illuminated check engine light. These symptoms are possible but some may not show up at all or will not be strong enough to be noticeable:
- Poor fuel economy
- Possible engine misfire
- Faulty oxygen sensor leak in exhaust portion that is close to the sensor.
- Oxygen sensor signal circuit is shorting out.
- Corrosion in oxygen sensor’s connector, which causes open in circuit resistance.
What to Check
- Locate the oxygen sensor 2 in bank 1.
- First, the technician should monitor the oxygen sensor’s data to verify the problem and confirm that the voltage is too low.
- Next, check the oxygen sensor’s writing and connectors for evidence of corrosion. Corrosion will interfere with the oxygen sensor’s ability to work properly.
- Also, be sure to check the oxygen sensor for any contaminants (like oil or coolant) that may have been caused by engine leaks. If this is the case, clean the oxygen sensor and replace it.
- Check the oxygen sensor’s heater circuit to ensure the resistance is within specifications.
- Verify that there are a 12-volt heater signal and a good ground connection to the sensor.
- Check the catalyst as well, in front of the oxygen sensor. It will not operate properly if the catalyst is damaged.
- Check for exhaust leaks in front of the oxygen sensor (remember this code refers to the second sensor in bank 1).
How to Fix It
- Replace the second oxygen sensor for bank 1.
- Repair or replace the wiring and/or connectors for bank 1, sensor 2.
- Repair any possible exhaust leaks.
- If checking all the electrical connections produces no consistent, verifiable results, then remove the oxygen sensor. If you notice that it has a chalky, white appearance, then this means it has been lagging between switching phases and needs to be replaced. A healthy oxygen sensor should be light tan in color, like a spark plug.
- To prevent any incorrect readings, be sure to repair any exhaust leaks first.