Lexus: What Does Error Code P0138 Mean?
P0138 is the OBD-II generic code indicating that the O2 sensor for bank 1 sensor 2 fails to have a lower voltage output below 1.2 volts for more than 10 seconds indicating a lack of oxygen in the exhaust stream.
This article applies to the Lexus IS, RX, GS, LS, and ES (2005-Present).
Every Lexus vehicle comes equipped with an OBD connector which can be used to send messages to an OBD2 troubleshooting reader like the one below. Simply connect the scanning tool (or a computer with scanning software downloaded) to the connector (located under the steering wheel) and the readout will show you Diagnostic Trouble Codes that might be affecting your car. From there, you can look up the code at ClubLexus.com to learn what's going on and try to repair it yourself. If you are not comfortable diagnosing and repairing your car, please take it to a Lexus service center. Scanner programs and tools vary in price. An app can be purchased for $4.95, and an OBD-II Scanner/Code Reader can be purchased for anywhere between $19 and $300+ depending on what service functions you need.
P0138 Definition
P0138 is the OBD-II generic code indicating that the O2 sensor for bank 1 sensor 2 fails to have a lower voltage output below 1.2 volts for more than 10 seconds indicating a lack of oxygen in the exhaust stream.
P0138 Explained
The job of the rear oxygen sensor is to monitor the performance of the catalytic converter. If the gases leaving the catalytic converter have low oxygen content, this can be an indication that it's wearing out. A properly operational catalytic converter should store exhaust gas oxygen so it can be used as a 'catalyst' to complete the combustion process of any unburned exhaust gases leaving the engine.
The purpose of the oxygen sensor is to measure the oxygen content in the exhaust gases after they leave the combustion process of the engine. This data is critical in enabling your vehicle to produce the best power while simultaneously expelling the least amount of air pollution. The air-fuel ratio in your Lexus must be maintained near the ideal stoichiometric ratio of 14.7:1, which means 14.7 parts of air (or oxygen) to 1 part of fuel. If there is too little oxygen in the exhaust (and your sensor detects a voltage level above 0.45 V), your engine is running too rich and using excessive fuel. This wastes fuel and pollutes the air with carbon monoxide. When this occurs, the Power Train Control Module or PCM will cut back on the amount of fuel it delivers to the engine. If there is too little oxygen in the exhaust (and your O2 Sensor detects a voltage level below 0.45 V), your engine is running too lean and is polluting the air with poisonous nitrogen oxides and raw hydrocarbons. When this occurs, the PCM will increase the amount of fuel delivered to the engine.
P0138 Symptoms
- Check Engine Light will illuminate
- The vehicle may run lean or rough (see explanation above)
- The engine may hesitate or misfire
- You notice a decrease in fuel economy
- Bad smelling exhaust
- In some unusual cases, there are no adverse conditions noticed by the driver
P0138 Common Causes
- Faulty O2 Sensor
- Defective O2 Sensor/air-fuel ratio sensor heater circuit
- Exhaust system leak
- Defective catalytic converter
- Intake air system leak (including vacuum leaks)
- Low fuel pressure
- Defective engine coolant temperature sensor
- Defective sensor wiring and/or circuit problem
- PCM software needs to be updated
- Defective PCM
- 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
P0138 Diagnosis and Repair
The number one most common repair for this problem is replacing the O2 Sensor itself. Another repair would be to replace the wiring or connection for Bank 1, Sensor 2. If the catalyst is the problem, replace the catalytic converter. Replace the fuel injector if it is leaking.
- Check around the O2 sensors for visual possible corrosion that might need to be cleaned.
- Check for damage to exhaust pipes and damage to the catalytic converter.
- Check live data from all 4 terminals of O2 sensors. Make sure your key is on and your engine is off when checking this. You are checking for ground and signal reference. As long as you are getting ground and .3 volts of signal reference.
- Replace O2 Sensor
- Repair short to battery voltage in O2 Sensor signal circuit
- If these steps do not clear your Check Engine Light, you may need to visit a mechanic because there may be internal electrical issues.
Final Notes
It is okay to drive a vehicle with P0138 for a short period of time, but driving with this code for an extended period of time can overheat the engine and cause internal engine damage.
After repairs have been made, clear stored codes from the ECU and test drive the vehicle between 30-35 miles per hour for about two miles. If the O2 Sensor signal ever goes to a negative voltage, or in excess of one volt, this is an indication that the sensor is bad. These inconsistent readings are often the reason to believe the circuit is losing voltage or ground into the O2 Sensor circuit. If the O2 Sensor body has a chalky appearance, this indicates lag between phases and should be replaced.
