Keeping the air/fuel mixture correct relies on a hot, clean oxygen sensor to give the ECM the feedback it needs for closed loop operation.
It’s a brutal environment. 1600-degree heat, vibrations, noxious exhaust fumes, corrosion and neglect. If it sounds like a job from Hell, in a sense it is and oxygen sensors endure this hellish environment for years without complaint. While that’s good news for owners, when the sensors do act up, the neat, clean closed loop dance orchestrated by the ECM breaks down immediately. The trouble is, “Check Engine” isn’t particularly helpful, and sometimes there isn’t even the “idiot light” to signal trouble.
How they work
O2 sensors are one of those technologies whose results are easy to understand, but the way they derive the data isn’t. Without the materials science, oxygen sensors are specialized ceramic electric cells that can generate a voltage when there is a difference in oxygen concentration across the cell. It’s a handy feature since O2 concentration difference between the exhaust gas and the outside or “ambient” air is another way of measuring the effective fuel-air ratio in the cylinders. Typical voltages range from zero to just over a volt with an average of 0.45 volts. In practice, once the sensor starts operating (about 600F, which is why they’re mounted close to the manifold, inside double wall pipe, or are electrically hearted) it will oscillate around the 0.45 volt mean, which manufacturers call “O2 cross counts”. The fact that the sensor doesn’t spend time actually at the mean value makes it easy for the ECM to detect just when the sensor is warm enough to allow the switch to closed-loop operation. Many vehicles do this by sending a steady 0.45 volt “bias” signal down the sense circuit, which the ECM software is programmed to recognize as a sensor that’s either too cold or is non-functional. The higher the number of O2 cross counts the more responsive the sensor.
Testing for repeatability
Testing oxygen sensors isn’t difficult, but to get repeatable results, a few basic factors are important to remember. Leaded fuel isn’t the problem it used to be, but if the owner is using octane-boosting additives or upper cylinder lubes in the fuel, contamination is still a possibility. If possible, ask the owner or make a note about contamination so the service advisor can make an additive recommendation. The same goes for another do-it-yourself sensor killer: RTV silicones. “Sensor-Safe” isn’t an advertising gimmick but a necessity wherever the sealant is exposed to the engine crankcase. That includes the popular valve cover “gasket in a tube” home fix that’s advertised by the RTV manufacturers.
The other contaminant that attacks oxygen sensors from inside the pipes is the fuel itself, or more accurately, excessively rich mixtures. Rich usually equates to low exhaust gas temperatures and high hydrocarbon emissions, as well as lower NOx, so a four or five-gas analyzer and your nose are viable diagnostic aids.
Oxygen sensors have a port to ambient air, so anything in the underhood environment that works it’s way down into the area can plug the orifice and cause erratic operation or worse. Oil and coolant are obvious fluids, but another possibility is undercoating. For most technicians, driveability or emission troubles after rustproofing doesn’t come immediately to mind, but it’s worth considering if the goo is too liberally applied to the floor pan.
Some technicians use sophisticated test equipment for O2 sensors, but many prefer a digital multimeter or analog VOM. One myth that’s worth dispelling is the notion that improper use of a voltmeter can damage the sensor. Measuring O2 sensor output is harmless, although older analog voltmeters with low “input impedance” might load the system enough to give artificially low readings. A million ohms per volt is the target impedance, which most digital meters reach with ease. Analogs, however, are only this good at higher price points. If you’re not sure about your analog, check the meter face (or manual) for the specification.
Shorting the wiring to ground is usually harmless, simply completing the low-current sense circuit locally instead of through the ECM. Shorting the heater circuit, however, will have consequences, but not normally to the sensor itself.
The same isn’t true, however, if you’re attempting to measure resistance across the heater circuit. Multimeters and VOM’s use an internal power source to measure resistance using Ohm’s Law, and if the only resistance in the circuit is the sensor’s heater, then damage is a possibility from excess current. A good oscilloscope is an ideal tool for O2 sensor work since the ability to trade waveforms along with voltage lets the technician look at the cross counts. In closed loop operation, there should be several cross counts per second, so set the time base appropriately, or this valuable information can be lost. Using the ‘scope during warm up shows the response of the ECM to the sensor as it warms into the operating range. It takes several cross counts to trigger closed loop operation, so a “memory” feature can be handy.
There’s much more to say about oxygen sensor operation and how it relates to the rest of the emission control system, but the same rules apply as with any engine/driveability service: Check the simple things first, know the system well, install quality parts and oxygen sensor service will be a low comeback, high customer satisfaction moneymaker. SSGM
Testing OBD II vehicles for O2 Operation
OBD II equipped vehicles have remarkable self-diagnostic capabilities, but to get a true code, the correct driving cycle should be followed to avoid chasing phantom problems and needless parts replacement. The procedures vary, but the following General Motors checklist is typical:
1. Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 50C (122F) and within 6C (11F) of the ambient air temperature at startup. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run.
2. Idle. The engine must be run for two and a half minutes with the air conditioner on and rear defroster on. The more electrical load you can apply the better. This will test the O2 heater, Passive Air, Purge “No Flow”, Misfire and if closed loop is achieved, Fuel Trim.
3. Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 88km/hr (55mph) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.
4. Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for 3 minutes. During this time the O2 response, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.
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