December 2005 Articles & Features
By Bob Pattengale | December 2005
Air/fuel ratio sensors facilitate fuel control that is more precise than is possible with conventional zirconia oxygen sensors. Interpreting signals from the newer sensors requires a change in both tools and techniques.
Vehicle manufacturers face many challenges when producing automobiles. One of the greatest is balancing the consumer's desire for high performance with the government's mandate to keep the air clean. The demands on engineers to reduce tailpipe emissions, increase fuel economy and improve engine performance drive the creation of many new technologies.
Fuel and engine control management systems have evolved significantly over the years and are tasked with performing that balancing act. The oxygen sensor is one of the key engine management system components. The conventional zirconia dioxide O 2 sensor is widely used and responds to changes in the exhaust content. It switches from approximately 0 volts (lean) to 1.0 volt (rich) during normal operation. It also provides feedback to the powertrain control module (PCM) to maintain a close to stoichiometric (14.7:1) air/fuel ratio. This feedback is important to the three-way catalytic converter, which needs the stoichiometric ratio to operate efficiently.
The zirconia dioxide oxygen sensor has remained basically unchanged for nearly 30 years. But one change that was made was the addition of a sensor heater. The oxygen sensor needs a temperature of approximately 650°F to operate properly. The heater is used to reduce the time to closed-loop during cold-start conditions and to maintain closed-loop at idle during lower exhaust temperatures.
Fuel control feedback using conventional zirconia dioxide O 2 sensors is very narrow and constantly changing. If a mixture other than stoichiometry is desired, the PCM will likely go into open-loop operation. To achieve greater fuel control management under any operating condition, the PCM needs to know the exact fuel mixture at any given time. The next-generation O 2 sensors, primarily called air/fuel ratio sensors (A/F ratio sensors), are designed to achieve this goal. Air/fuel ratio sensors allow for closed-loop fuel control at mixtures other than stoichiometry.
Single-Cell & Dual-Cell Air/Fuel Sensors
Air/fuel ratio sensors may be referred to as wide-range, wide-band, linear, lean, etc. There are two basic sensor designs�single-cell and dual-cell. Single-cell sensors generate voltage in a manner similar to a conventional O 2 sensor. They're used to control current flow through a specialized balanced control/monitoring circuit within the PCM.
Toyota uses a single-cell linear A/F ratio sensor. The Toyota air/fuel sensor looks similar to the conventional zirconia dioxide O 2 sensor; both have four wires and the connectors look the same. A zirconia sensor can be plugged into the A/F ratio sensor harness with minor modifications; however, the system will not work properly with an incorrect sensor.
Dual-cell sensors typically use the planar design and are sometimes referred to as current-pumping or ion-pumping sensors . They can be either wide-range or lean air/fuel ratio sensors. Honda's lean-burn engines use the lean A/F ratio sensor and can respond to extremely lean mixtures�approximately 22:1. Wide-range A/F ratio sensors have an effective air/fuel ratio range of approximately 12:1 to 20:1. Both the single-cell and dual-cell sensors require higher temperatures (approximately 1200°F) to operate properly.
Single-cell linear air/fuel ratio sensors found on some California-certified Toyota vehicles starting in 1996 can cause some confusion. The confusion is how the sensor is reported on an OBD II generic scan tool. Fig. 1 (above) is a photo of the screen of the OTC Genisys scan tool demonstrating the B1 S1 O 2 sensor (the sensor parameter is highlighted in blue). In order to comply with the OBD II scan tool regulations, Toyota decided to report the air/fuel ratio sensor in a normalized voltage, 0 to 1.0 volt. The chart in Fig. 2 explains how the normalized voltage works. The Genisys scan tool shows the voltage at .64 volt. This indicates the current air/fuel ratio is approximately 14.4:1, or slightly rich. The enhanced factory scan tool reports the linear A/F ratio sensor as AF B1 S1, with a voltage ranging from 2.4 to 4.0 volts. In this case, the voltage on the factory scan tool would be 3.2 volts.
The confusion really begins when a technician attempts to test the sensor. If you were not aware that this vehicle was equipped with an air/fuel ratio sensor, you would follow conventional zirconia sensor testing procedures. You'd expect the sensor to sweep up and down a range of 0 to 1.0 volt. However, in this case, the sensor would remain steady at approximately .66 volt. The next step would be to verify if the sensor can register a lean mixture below .2 volt and a rich mixture above .8 volt. In this case, if you created a lean condition with a vacuum leak, the voltage would increase to approximately .8 volt, then return to approximately .66 volt once the vacuum leak was removed. You'd expect the voltage to decrease to below .2 volt. Creating a rich condition using propane would cause the voltage to decrease to approximately .48 volt. Once again, you'd expect the voltage to increase to above .8 volt.
At this point, most technicians might think the oxygen sensor or maybe even the PCM should be replaced. The California Air Resources Board (CARB) saw the potential issues this might create and approved a new OBD II generic parameter for A/F ratio sensors. Toyota made the correction in later model vehicles. On these vehicles, if your OBD II generic scan tool does not include the new OBD II generic parameters, the scan tool will not display a B1 S1 value. Depending on the type of A/F ratio sensor used, the parameter may be displayed in amps or voltage.
Toyota recommends testing a linear air/fuel ratio sensor with the factory scan tool. Fig. 3 (on page 42) shows several captures from the Mastertech scan tool. Using the A/F Control bidirectional test, the fuel mixture was commanded lean, then rich. The last line in the first screen capture shows the mixture being commanded 12% lean. The AFS B1 S1 parameter reads 3.92 volts, which indicates a lean condition�approximately 18.4:1. The middle capture shows the mixture commanded to 0%. The AFS B1 S1 parameter returned to 3.43 volts�slightly lean, but close to stoichiometry. The final capture shows the mixture commanded 25% rich. The AFS B1 S1 shows 1.83 volts, which is off-the-chart rich. Based on this information, the A/F ratio sensor tested normal.
The air/fuel ratio sensor can be tested without the bidirectional control by manually creating a rich/lean mixture and watching the voltage. If the air/fuel ratio sensor voltage does not change or the sensor is slow to react, the next step is to check the operation of the sensor heater. Fig. 4 (above) shows one option for testing the sensor manually. The four-wire sensor has two wires for the heater and two for the sensor signal. Fig. 5 shows a typical linear A/F ratio circuit. The heater circuit B+ wire should show battery/charging system voltage. The sensor circuit can be tested with a low-current probe and lab scope. The low-current probe can be used on either heater wire. Fig. 6 is what you'd expect to see from a normal sensor on a lab scope. The heater circuit is duty cycled to control the current and the peak amperage should be approximately 6 amps, depending on the application.
There are several methods for checking the sensor signal circuit. One involves using a voltmeter and checking each wire. One wire should have 3.0 volts and the other 3.3 volts. If the voltage at either wire is different from those numbers, check for open or shorted circuits or possible PCM issues. A second method involves disconnecting the sensor signal wires and measuring the voltage across the sensor, while driving the mixture rich, then lean. The sensor will function like a conventional O 2 sensor, ranging in voltage from 0 to 1.0 volt. Keep in mind that the heater circuit must remain connected to maintain the proper sensor temperature. The third option for checking the sensor signal circuit requires connecting a DVOM in series to one of the signal wires. Using the mA portion of the DVOM, watch the current flow as you drive the fuel mixture rich, then lean. The current flow should switch from positive to negative as the mixture swings.
Fig. 7 shows DTC and freeze frame data from a 1998 Toyota Camry. The vehicle had two recurring fault codes�DTC P0125 (Insufficient Coolant Temperature for Closed Loop Fuel Control) and P1135 (A/F Sensor Heater Circuit Malfunction, not displayed here). The P1025 code is confusing due to the mention of insufficient coolant temperature. This code has nothing to do with engine coolant temperature. The DTC will set if the sensor output shows little or no activity for 90 seconds. The following conditions need to met for the monitor to run: engine speed of 1500 rpm or more, vehicle speed of 25 to 62 mph, the TPS must not show idle, these conditions must continue for at least 90 seconds and the engine must run for at least 140 seconds.
The P1135 code is the factory version of P0125, and the diagnostic process is similar. The most common cause is a faulty sensor. Fig. 8 shows 60 seconds of a road test following sensor replacement, and demonstrates that the sensor is operating properly. A second option for verifying repair effectiveness is to check Mode 6 data. In this case, the correct conditions must be met for the test to run. Then you'd need to check the Test Identification (TID) $07 Component Identification (CID) $01 following a road test.
Current Pump Senors
The Bosch LSU 4 is a wide-range air/fuel ratio sensor and is often referred to as a current pump sensor . It's comprised of two zirconia cells, with one of the cells acting as an input to the PCM. The pump cell acts as an ion pump, pumping oxygen ions from one side of the sensor to the other. The PCM monitors the Nernst signal line and attempts to keep the voltage at .45 volt. The PCM will increase and decrease the current flow to the pump cell to maintain that voltage level. A trimming resistor is built into the sensor harness to compensate for manufacturing tolerances. Its range is 30 to 300 ohms and varies from sensor to sensor. The heater circuit is similar to the linear A/F ratio sensor and is critical to proper sensor operation.
Once again, the best way to diagnose this A/F ratio sensor is with a scan tool. If one is available, an exhaust gas analyzer can be combined with a scan tool to verify sensor accuracy. Fig. 9 (on page 45) shows screen captures from a scan tool and a gas analyzer. If the air/fuel ratio parameter value in the scan tool does not match the lambda value on the gas analyzer, additional diagnostics will be required.
Fig. 10 shows a simplified Bosch LSU 4 sensor circuit. Diagnosing it is simple and can be done in a few minutes. The red numbers in Fig. 10 show the basic four-step diagnostic process:
1. Check the heater circuit. As with the linear sensor, a current probe can be used to check for proper current. The minimum current value varies; check service information for specific values. Mode $06 will often supply minimum heater current values.
2. Disconnect the sensor harness connector. Using a DVOM, check the resistance of the trimming resistor across the input pump and output pump current wires. The value should be 30 to 300 ohms.
3. Reconnect the sensor connector and check the reference ground using a DVOM. The voltage typically should be 2.4 to 2.7 volts. (Refer to the manufacturer's specifications for exact voltages).
4. Check the pump cell and Nernst signal. Using a dual-channel scope, connect the scope ground to the reference ground. Next, connect one channel of the scope to the Nernst signal and the other channel to the input pump signal. The Nernst signal voltage should remain steady at .45 volt under all conditions. If the voltage is not .45 volt, check the circuit first, then check the input pump cell. Pump cell voltage should vary about .5 to .6 volt, switching above and below 0 volts. The scope needs to be set in bipolar or AC-coupled mode for you to be able to see the negative swings. Driving the fuel mixture rich, then lean should produce a change in the voltage of greater than 1.0, similar to a conventional O 2 sensor.
The key to a quick and accurate diagnosis is having the right tools for the job. Enhanced scan tools provide the greatest source of diagnostic information, and an exhaust gas analyzer with lambda also can help with diagnostic accuracy. The use and refinement of air/fuel ratio sensors will continue as long as the demands for fuel efficiency, cleaner air and improved performance drive the industry.