1998 Land Rover Discovery Series I OBD2 Code P0138 O2 Sensor Circuit High Voltage

What Does P0138 Code Mean?

Code P0138 is triggered when the powertrain computer or PCM determines that the rear oxygen sensor voltage remained above 450 millivolts for more than twenty seconds (varies with vehicle make and model) or that the air fuel ratio sensor remained in a rich-biased mode for too long (varies with vehicle make and model).

The job of the rear oxygen sensor is to monitor the performance of the catalytic converter(s) . If the gases leaving the catalytic converter has 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 vital in order for the engine to produce the best power while at the same time, producing the lowest possible amount of air pollution. If there is too little oxygen in the exhaust, it means the 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, this means that the 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
• Vehicle may idle or run rough
• Decrease in fuel economy
• Bad smelling exhaust
• In some unusual cases, there are no adverse conditions noticed by the driver

Common Problems That Trigger the P0138 Code

• Defective oxygen sensor/air fuel ratio sensor
• Defective oxygen 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

Polluting Gases Expelled

• HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
• CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
• NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog

Diagnostic Theory for Shops and Technicians: Oxygen Sensor

When the code P0138 is set, record the freeze frame data in fine detail. Next, duplicate the code setting conditions on a test drive, paying particular attention to load, MPH, and RPM. The best tool to use on this test drive is a data streaming scan tool that has factory quality and dedicated live data. Be sure to verify the code conditions before you advance to the next set of tests.

If You Cannot Verify the Code Setting Malfunction

If you cannot verify the code setting malfunction, then do a careful visual inspection of the sensor and the connections. Verify that there are 12-volt heater signal(s) and good ground(s) to the sensor and that they follow the required times, per the manufacturer diagnostic documentation. Verify that the signal from the oxygen sensor to the PCM is being "seen" by back probing the oxygen sensor connector and, if needed, back probing the signal wire at the PCM. Inspect the sensor harness to ensure that it isn't chafed and/or grounding anywhere and be sure to perform a wiggle test. You will want to use a high impedance Digital Volt Ohm Meter (DVOM) for all of these electrical tests. If you still cannot find a problem, then try these steps next:

• If you can receive authorization from the customer to keep the vehicle overnight, clear the code and test drive the vehicle by driving it home and then back to work in the morning, making sure that you are duplicating the code setting driving conditions on both trips. If the code still does not come back, you can give the customer the option of replacing the oxygen sensor as a diagnostic step since the sensor is the most likely problem and the code will presumably set again. If the customer declines, then return the vehicle with a clear description of the inspections and your findings plainly attached to the final copy of the repair order. Keep another copy for your own records in case you have to re-visit this inspection for any reason.
• If this is an inspection for an emissions failure, most government programs suggest that you replace the sensor as a preventative measure so the vehicle won't remain in a highly polluting operational condition. After the oxygen sensor is replaced, the monitors will have to be re-set and this, too, will test most phases of the oxygen sensor system to ensure that the problem was solved. Be sure to verify that the Mode 6 test IDs and component IDs that pertain to fuel control are well within the parameter limits. If there is a problem with re-setting the monitors, continue the inspection until you find the root cause of the problem.

If You Can Verify the Code Setting Malfunction

If you can verify the code setting malfunction, then do a careful visual inspection of the sensor, the connections, and the exhaust system. Make sure that there are no exhaust leaks upstream of the oxygen sensor. Verify that there are 12-volt heater signal(s) and good ground(s) to the sensor and that they follow the required times, per the manufacturer diagnostic documentation. Verify that the signal from the oxygen sensor to the PCM is being "seen" by back probing the oxygen sensor connector and, if needed, back probing the signal wire at the PCM. Inspect the sensor harness to ensure that it isn't chafed and/or grounding anywhere and be sure to perform a wiggle test. You will want to use a high impedance Digital Volt Ohm Meter (DVOM) for all of these electrical tests.

• The most comprehensive way to test and condemn an oxygen sensor heater circuit is to use a Dual Trace Labscope with the time division graticule set at 100-millisecond intervals and the voltage scale set at +/- 2 volts. Run the warmed-up vehicle with the signal wire back probed and watch to see if the signal sticks and for how long. Do this while the engine is idling and at 2000 RPM. A properly working oxygen sensor should switch from lean (less than 300 millivolts) to rich (above 750 millivolts) in less than 100 milliseconds and should do it consistently.
• Next, perform a range test and time test, still using the Labscope. Run the engine at 2000 RPM and quickly close the throttle and then snap it back open. The oxygen sensor signal needs to go from around 100 millivolts (when the throttle closes) to above 900 millivolts (when the throttle opens) in less than 100 milliseconds. A new sensor will do this test within these ranges in less than 30-40 milliseconds.
• If the sensor fails either of the above Labscope inspections, most emissions programs will allow you to condemn the sensor because the slow switching time leads to high NOx levels and above-normal CO levels and HCs. This is because the Cerium bed of the OBD II Catalytic Converter is not being supplied with the proper amount of Oxygen each time the signal "lags" between the peaks and valleys of its sine wave.
  

Note:
If the oxygen sensor signal ever goes to a negative voltage or above 1 volt, this alone is enough to condemn the sensor. These out-of-range readings are often caused by the heater circuit bleeding voltage or ground into the oxygen sensor signal circuit. They can also be caused by contamination or physical damage to the sensor. - If the above tests and inspections don't produce verifiable results, then physically remove the oxygen sensor. If the sensor probe has a white and chalky appearance, the sensor has been lagging between switching phases and needs to be replaced. It should have the light tan coloration of a healthy spark plug .

P0138 Diagnostic Theory for Shops and Technicians: Air Fuel Ratio Sensor

Most air fuel ratio sensors are basically two heated oxygen sensors that work in tandem in order to create a much faster responding oxygen sensor/fuel control system. These systems are also capable of "'broadband" operation, which means that the vehicle will remain in closed loop and maintain active long term and short term fuel control during wide open throttle conditions. A conventional oxygen sensor system cannot maintain fuel control when the throttle is above 50 percent and the vehicle is under heavy load, such as wide open throttle.

When the code P0137 is set, record the freeze frame data in fine detail. Next, duplicate the code setting conditions on a test drive, paying particular attention to load, MPH, and RPM. The best tool to use on this test drive is a data streaming scan tool that has factory quality and dedicated live data. Be sure to verify the code conditions before you advance to the next set of tests.

If You Cannot Verify the Code Setting Malfunction

If you cannot verify the code setting malfunction, then do a careful visual inspection of the sensor and the connections. Verify that there are 12-volt heater signal(s) and good ground(s) to the sensor and that they follow the required times, per the manufacturer diagnostic documentation. Verify that the signal from the oxygen sensor to the PCM is being "seen" by back probing the oxygen sensor connector and, if needed, back probing the signal wire at the PCM. Inspect the sensor harness to ensure that it isn't chafed and/or grounding anywhere and be sure to perform a wiggle test. You will want to use a high impedance Digital Volt Ohm Meter (DVOM) for all of these electrical tests. If you still cannot find a problem, then try these steps next:

• If you can receive authorization from the customer to keep the vehicle overnight, clear the code and test drive the vehicle by driving it home and then back to work in the morning, making sure that you are duplicating the code setting driving conditions on both trips. If the code still does not come back, you can give the customer the option of replacing the oxygen sensor as a diagnostic step since the sensor is the most likely problem and the code will presumably set again. If the customer declines, then return the vehicle with a clear description of the inspections and your findings plainly attached to the final copy of the repair order. Keep another copy for your own records in case you have to re-visit this inspection for any reason.
• If this is an inspection for an emissions failure, most government programs suggest that you replace the sensor as a preventative measure so the vehicle won't remain in a highly polluting operational condition. After the oxygen sensor is replaced, the monitors will have to be re-set and this, too, will test most phases of the Oxygen Sensor system to ensure that the problem was solved. Be sure to verify that the Mode 6 test IDs and component IDs that pertain to fuel control are well within the parameter limits. If there is a problem with re-setting the monitors, continue the inspection until you find the root cause of the problem.

If You Can Verify the Code Setting Malfunction

If you can verify the code setting malfunction, then do a careful visual inspection of the sensor, the connections, and the exhaust system. Make sure that there are no exhaust leaks upstream of the air fuel ratio sensor. Verify that there are 12-volt heater signal(s) and good ground(s) to the sensor and that they follow the required times, per the manufacturer diagnostic documentation. Verify that the signal from the oxygen sensor to the PCM is being "seen" by back probing the oxygen sensor connector and, if needed, back probing the signal wire at the PCM. Inspect the sensor harness to ensure that it isn't chafed and/or grounding anywhere and be sure to perform a wiggle test. You will want to use a high impedance Digital Volt Ohm Meter (DVOM) for all of these electrical tests.

There are numerous, complex tests for an air fuel ratio sensor, but these are the simplest and most time-efficient tests:

• Air fuel ratio sensors may have several wires, but there are two key wires. Using a DVOM with the key on and the engine off, disconnect the sensor and probe the harness going to the PCM. Make sure one wire has 3.0 volts and another wire has 3.3 volts. The other wires are the 12-volt power(s) and ground(s) for the heater circuits. In some cases, you may have to start the engine and let it idle to find the proper voltages on all the wires.
• Use jumper wires to connect the sensor to the harness. Connect your DVOM in series with the 3.3 volt wire. Turn your DVOM to the milliamp scale and start the engine, letting it idle. The 3.3 volt wire should cross-count between +/- 10 milliamps. Vary the RPM and as you add and decrease throttle, you should see the signal respond to subtle changes in mixture. If you don't consistently see the +/- 10 milliamp variation in this wire, then the Air Fuel Ratio Sensor is defective.
• If all the above tests and inspections do not produce verifiable results, then physically remove the air fuel ratio sensor. If the sensor probe has a white and chalky appearance, the sensor has been lagging between switching phases and needs to be replaced. It should have the light tan coloration of a healthy spark plug.