Diagnostic Solutions: Air/Fuel Ratio Sensors – UnderhoodService

Diagnostic Solutions: Air/Fuel Ratio Sensors

Diagnose and Solve the Root Cause of Sensor Failure Before Sensor Replacement

Diagnosing a fuel control system can be confusing for import technicians because air/fuel ratio (AFR) sensors used in many imports today operate on an entirely different principle than do the conventional narrow-range oxygen sensors. The first step in any fuel control sensor diagnosis is to distinguish whether the scan tool data is being generated by a conventional, narrow-range oxygen sensor or by the more modern “wide-range” air/fuel ratio (AFR) sensor.

Narrow-range zirconia sensors must reach at least 600° F to generate a voltage signal. When reading a cold sensor with the key on, engine off (KOEO), the scan tool should show a bias voltage representing mid-range (about 0.45 volts) on the applicable type of sensor. Some PCMs may also generate a “reference” voltage of about 5 volts if the oxygen sensor suffers from an open circuit in the sensor or wiring. As a narrow-range or “zirconia” oxygen sensor reaches an operating temperature of about 600° F, it’s designed to switch voltages from “lean” at about 0.2 volts, to “rich” at about 0.8 volts. A “stoichiometric” 14.7:1 air/fuel ratio occurs at approximately 0.5 volts, which is mid-point on a 0-1 volt narrow-range zirconia sensor. The zirconia sensor’s primary disadvantages are that it can measure only a very narrow band of air/fuel ratios and cannot correct air/fuel ratios as rapidly as can an AFR sensor.

Although titania oxygen sensor applications are relatively rare, it’s important to know that titania sensors used in some imports are essentially resistors that ground a reference voltage from the PCM. Most operate in a 0-5 volt or 0-1 volt range. In the case of the 5-volt titania sensor, “stoich” occurs at about 2.5 volts, with lower voltages representing rich and higher voltages representing lean.

Because lowering exhaust emissions and increasing fuel economy continues to be the aim of import manufacturers, a much wider range of air/fuel ratios must now be measured than is possible with the conventional oxygen sensor. Since I can’t describe the exact chemistry and physics of how an AFR sensor works due to space limitations, suffice it to say that two basic designs are now used in import and domestic vehicles and both utilize different operating principles. Both, however, generate a similar signal for the PCM to process.

With either sensor, however, the engine’s PCM measures air/fuel ratios by monitoring a very small current that flows through the AFR sensor. A stoichiometric or chemically correct air/fuel ratio of 14.7:1 generates zero current flow through the AFR sensor. If the mixture is rich, a negative current flow is returned to the PCM. A lean mixture returns a positive current flow to the PCM.

When observing scan tool data, remember that the AFR sensor doesn’t switch the voltage signal from lean to rich as does a conventional oxygen sensor. The air/fuel ratio commanded by the PCM is very accurate and, unless a problem occurs that causes a rich or lean fuel mixture, the voltage parameter on the scan tool remains constant at constant engine speeds. A Toyota AFR sensor, for example, produces 3.3 volts at a stoichiometric ratio. A rich mixture produces a lower voltage, while a lean mixture produces a higher voltage.

AFR sensors are located in the “pre-catalytic” converter or upstream position and a conventional narrow-range oxygen sensor is located in the “post-cat” or downstream position. The basic function of the upstream sensor is to maintain fuel control, while the basic function of the downstream sensor is to monitor the efficiency of the catalytic converter. The PCM monitors catalytic efficiency by comparing the data from the upstream sensor to that of the downstream sensor.

For some vehicle applications, the downstream sensor may also serve in a fuel control capacity. If the upstream sensor fails, the downstream sensor may act as a back-up fuel control sensor. In other operating conditions, the downstream sensor may actually act in concert with the upstream sensor to help control air/fuel ratios. This is a very application-specific feature and may only function in certain circumstances.

When diagnosing an OBD II fuel control sensor, it’s important to remember that the PCM is the final arbiter of oxygen sensor or AFR sensor malfunctions. In addition, fuel control functions might differ, even in identical vehicle platforms, because the PCM might be programmed or reprogrammed to react differently to input data from the AFR and zirconia oxygen sensors.

The functionality of an AFR, or any fuel control sensor for that matter, is measured by a math formula programmed into the PCM. The technician must, therefore, depend upon the PCM to accurately monitor AFR functions. Because the amount of current transmitted to and from the AFR sensor is so small, it’s impossible to measure it externally with conventional testing equipment. Inasmuch as the PCM is programmed to detect data indicating a faulty AFR sensor and set the appropriate diagnostic trouble code (DTC), using a scan tool is the most foolproof method for diagnosing OBD II fuel control sensor issues.

When diagnosing fuel control sensor issues, it’s important to understand the difference between “generic” and “enhanced” data. “Generic” or “Global” parameter identification data (PID) is an EPA-mandated format in which fuel control data is displayed using a common terminology and format for all import and domestic auto manufacturers.

Auto manufacturers use enhanced data for diagnosing application-specific issues on their own platforms. To add to the confusion, a technician might find that two different aftermarket scan tools, when placed side-by-side, might display enhanced fuel control data in two different formats. So, for the sake of simplicity, I’ll refer to fuel control data in the generic format.

Short-term fuel trim (SFT) and long-term fuel trim (LFT) data are the most common parameters used to diagnose fuel control issues. In general, both SFT and LFT should be as close to zero as possible. The maximum variation from zero is generally 5% and a “rich or lean” trouble code might be stored if the value exceeds 25% for a specified length of time.

Remember that an extremely lean air/fuel ratio caused by a faulty air flow sensor or major vacuum leak can cause the fuel control sensor to become inactive due to an over-abundance of oxygen in the exhaust stream. Similarly, an extremely rich air/fuel ratio caused by a mechanical failure like excessive fuel pressure or a ruptured fuel pressure regulator diaphragm can cause the fuel control sensors to become inactive by coating the sensors with thick black soot.

Fuel control sensor inactivity caused by excess oxygen in the exhaust stream can be detected by metering some propane into the air intake. If propane causes the sensors to become active, the problem lies with a lean air/fuel ratio. If propane doesn’t make the sensors become active, remove at least one sensor to inspect for a carbon coating or contamination. Fuel control sensors may become contaminated by coolant or by excess oil entering the exhaust gas stream.

Remembering that the PCM is the final arbiter of fuel control sensor function, it’s important to know that when the fuel control sensor is replaced on some imports, the PCM might require reprogramming as specified by the applicable TSBs to prevent setting a false fuel control sensor trouble code in the future. Whatever the case, always make sure that the root cause of the sensor’s apparent failure is cured before replacing the sensor itself.

In my recent Diagnostic Solutions column (Charging System Diagnostics & Troubleshooting, April 2008 ImportCar, pages 44-48), I wrote in reference to the alternating voltage present in charging systems that, “A reading of 0.8 to 1.0 volts is normal.” This statement is incorrect. The statement should read, “A reading of 0.08 to 0.10 volts is normal,” which is a rough estimate of what the normal AC voltage should be in an average charging system under average conditions. I apologize for whatever inconvenience this error might have caused.

The maximum level of alternating current (AC) voltage present in a charging system should also be clarified. The past rule of thumb has generally been 0.250 AC volts. Equipment manufacturers now recommend 0.500 AC volts as a real-world limit. Some alternator rebuilders report that the presence of 1.0 AC volt isn’t unusual for heavy-duty charging systems.

While a higher level of AC voltage doesn’t necessarily affect the functionality of the alternator, it can interfere with the operation of a vehicle’s on-board electronics. Because higher accessory loads and higher-output alternators have increased the presence of AC voltage in the charging system, a technician shouldn’t be surprised if he discovers higher levels of AC voltage in future applications.

— Gary Goms cap This is a five-wire connector, which indicates a “chambered” AFR sensor design.

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