You’ve heard the saying, “Where there’s smoke, there’s fire?” Well, the part of that adage you never heard was, “If there’s too much smoke, then it wasn’t a very good fire.” That’s right, smoke and soot aren’t just dirty words, they’re the telltale signs of an inefficient burn. It’s true for a campfire as well as an internal combustion engine.

But when it comes to building a fire inside an engine cylinder, things can get pretty tricky. It’s a complicated process that depends on many variables: the outside air temperature and barometric pressure, engine temperature, whether you’re accelerating or decelerating, and the condition of the engine. To determine how well it did in computing the air/fuel mixture, the computer relies on post-combustion information supplied by several oxygen sensors. Theoretically, the computer could also use a hydrocarbon (HC) sensor to detect the level of unburned fuel. But oxygen sensors provide car manufacturers with a less expensive way to test for unburned HCs. For example, if there’s too much oxygen left in the exhaust, the computer might assume that the air/fuel didn’t ignite due to a spark plug misfire. Or, it can figure that the fire went out early due to lack of fuel. How does the computer determine which problem caused the high oxygen reading? By comparing the oxygen sensor readings taken before and after the catalytic converter.

The precious metals in a catalytic converter, platinum, palladium, rhodium, and cerium react with unburned fuel to complete the combustion process and reduce pollution. The reaction creates intense heat in the 1,200°F -1,500°F range. That heat burns off any left over HCs or other pollutants. So, if the computer sees a high oxygen content before the exhaust enters the converter and a low oxygen content after the converter, it assumes that the problem was caused by a misfire. In other words, something used up the oxygen between the inlet and outlet, and that something was the combustion of the fuel left over from the misfire.

If the engine has a significant misfire problem, the misfiring cylinders dump a lot of unburned fuel and air into the converter. That raises the heat levels inside the converter to upwards of 1,900°-2,500°F. At that point the metals in the converter begin to melt and the converter literally self destructs. A flashing “Check Engine” light is the warning sign that your catalytic converter is heading towards a nuclear meltdown.

Enough theory—How does an oxygen sensor actually work?

First, a little Naval history. Back before smart bombs, gunners on Naval destroyers had a really hard time calculating the aim on their big guns. Sure, they started off with basic geometry, but wind conditions and air density could throw off their aim. So they would measure how far off each hit landed. By overshooting and undershooting the target, they could fine tune their aim.

That’s exactly how an engine computer calculates air fuel mixture, by going back and forth between mixtures that are too lean and too rich. With each reading, the computer narrows its calculations until the mixture falls into a very tight range. As a result, an oxygen sensor is constantly reporting rich and lean mixtures to the computer. In fact, the flip-flopping between rich and lean occurs between 5-7 times per SECOND in a multi-port fuel injected vehicle.

Keep the flip-flopping requirement in mind as we learn how the sensor works.

The oxygen sensor works like a miniature electrical generator. The portion of the oxygen sensor that sits inside the exhaust pipe is shaped like a miniature light bulb. It is made out of ceramic zirconium and the outside of the bulb is coated with a porous layer of platinum. The inside of the bulb contains two strips of platinum. The power is generated by the differential between outside oxygen and exhaust oxygen levels. So, the sensor is actually comparing the level of oxygen inside the exhaust with the oxygen level in the outside air. If there’s little-to-no oxygen in the exhaust, the large differential generates 9/10 of a volt. But if the level of oxygen in the exhaust is identical to the outside air, there is no differential and the sensor generates 0 or .1 volt.

Remember the flip-flopping? Well, the midway point between 0 volts and .9 volts is 450 millivolts. If the computer sees a reading above 450mV, it interprets it as a RICH mixture. Anything below 450mV is considered a LEAN reading.

In order to generate these readings, the zirconium and platinum have to be hot. In the early 80’s, car makers relied on exhaust gas temperatures alone to heat up the sensor. On a cold winter’s day, that delayed the computer’s ability to compute air/fuel mixtures until the driver had gone a few miles. Until the sensor was at operating temperature, the air/fuel mixtures were calculated from default programming from the factory. Working off of factory settings is known as Open Loop. Running in Open Loop is like a carbureted car running with the choke closed. The engine is running very rich and polluting like crazy. As soon as the oxygen sensor heats up to operating temperature and is able to report reliable readings, it’s considered “in the loop”, or Closed Loop. As emissions standards have tightened up, car makers have been forced to add built in heaters to their oxygen sensors. The oxygen sensor heater in a late model car can bring a sensor into Closed Loop status within 15 seconds. More on heaters later.

What can go wrong?

Because oxygen sensors sit in the exhaust stream, they’re subject to all kinds of abuse and contamination. A backfire, for example, can shatter the delicate ceramic bulb. A head gasket coolant leak can coat the porous platinum coating with coolant. Worn rings can coat the bulb with oily soot. If you’ve used household kitchen or bath silicone calk or silicone spray anywhere near the inside the engine compartment you can poison a sensor within just a few minutes. If you want to seal something on your car with silicon sealer, make sure the package says “sensor-safe”. If you have any of these engine problems, or use any non-sensor-safe silicone materials in the engine compartment and then get an oxygen sensor trouble code, you can bet that you’ve damaged your sensor.

Beyond those types of catastrophic failures, oxygen sensors last a fairly long time—between 60-100,000 miles. But they do eventually “wear out.” They also get “lazy,” reducing their flip-flopping rates from 5-7 per second down to 1-2. The computer simply cannot deal with a lazy sensor and will set a “Check Engine or Service Engine Soon” light.

Where to start your trouble shooting?

Many auto parts store will read trouble codes for free. But the biggest mistake do-it-yourselfers (DIY’ers) make when they get an oxygen sensor (O2) related trouble code is to assume that the sensor itself is bad. It may need replacing, but a lot of really good O2 sensors get tossed in the trash every year because DIY’ers and even some “professional” mechanics didn’t know how to interpret the readings or test the sensor. You will need a wiring diagram and either a digital multimeter, a scope, or a scanner to properly diagnose the problem. But before you start hooking up expensive test equipment, here are some troubleshooting tips to avoid replacing a good sensor:

Lack of switching can be the sign of a contaminated or dead sensor. But that’s rare. A more likely cause is a vacuum leak. In other words, the engine is sucking in outside air through a bad intake gasket or broken vacuum line AFTER the computer has already calculated its air/fuel mixture. With high levels of oxygen in the exhaust stream, the oxygen level sensor stops flip-flopping.

Possible causes of “lean” codes

1. Vacuum leak.
2. Oxygen sensor output wire has shorted to ground.
3. Clogged injectors that don’t deliver enough fuel.
4. Low fuel pressure.
5. Exhaust leaks, especially if they’re located near the oxygen sensor. Outside air gets sucked into the exhaust causing a false reading.

Troubleshooting a “Lean” code

Here’s how to diagnose a vacuum leak: Using just the hose from an automotive stethoscope or a piece of garden hose held up to your ear, move the hose around the intake manifold and listen for a “sucking” sound. Then check all vacuum lines for the same type of sound. Also, look for cracks in vacuum lines. Can’t hear anything? Then move on to test #2, the spray test. Use a can of aerosol carburetor cleaner and spray a fine mist along the intake manifold and vacuum hoses while the engine is running. Listen for any changes in RPM. The carb cleaner acts as a fuel and gets sucked into the leak, increasing idle speed. But be careful. Carb cleaner is flammable.

Another “lean” code diagnosis tip: If your car uses a MAF sensor located near the air filter box and has flexible duct from there up to the throttle body, check for cracks in the pleats of the flex duct. The computer calculates air/fuel mixture based partly on the amount and temperature of the air flowing through the MAF sensor. If the pleats of the flex duct are cracked and allowing air to get sucked in AFTER the sensor, the computer will conclude that the engine is running lean.

Connect a fuel pressure gauge to the fuel supply line and measure fuel line pressure. If it’s too low, check the fuel filter and the fuel pressure regulator for proper operation before you assume that the problem is with the fuel pump.

Troubleshooting a “Rich” code

Rich codes are triggered when combustion has burned all the available oxygen. Your job is to figure out whether the problem is caused by too much fuel, or too little air. A plugged air filter, for example, can restrict the flow of air so much that the computer sets a “rich” code. So checking the air filter should be first on your DIY troubleshooting list. A stuck injector can also cause a rich code because it allows too much gas into the cylinder.

Possible causes of “rich” codes

1. A leaking injector(s) can cause a rich exhaust.
2. High fuel pressure can deliver too much gas.
3. A malfunctioning canister purge system can pour fuel vapors from the gas tank into the intake air stream, causing a rich code.
4. Incorrect ignition timing.

Testing the oxygen sensor

As I mentioned earlier, late model cars have oxygen sensor heaters. So before you start attaching your test equipment, it’s important to have a wiring diagram to know which wires carry the voltage to the powertrain control module. If you opt to use a volt meter, make sure it is built for automotive use. Analog volt meters require too much current to move the mechanical needle and that current drain can fry your car’s computer. That’s why all automotive repair manuals specify a digital volt meter with at least 10 mega-ohms of resistance.

Attach the digital meter to the oxygen sensor and look for constantly changing voltage. If the reading does not change, you have a dead sensor, a full lean condition, or a full rich condition. If the meter is reading lean, add carburetor cleaner to the airstream and look for a changed reading. If the meter is reading full rich, pull of a vacuum line to induce an air leak. Then look for a change in readings.

How to replace the sensor

1. Purchase an oxygen sensor socket. You may be able to get the old sensor out by using conventional sockets, but it will be very difficult to install the new one without damaging the wiring harness.
2. Start the engine and heat up the exhaust pipe. You want it warm to the touch, but not hot enough to burn yourself. The heat will make removal much easier.
3. If the sensor will not budge, try heating the fitting with a propane torch. Then spray with cold water. The “shock” of the cold water may free up the sensor.
4. Sensors are available in “universal” and factory connector varieties. I recommend spending the few extra bucks and get the factory connector. If you decide you’d rather splice in the old connector, make sure you solder the connection and seal it with heat shrinkable tubing. Do NOT use a crimp connector to make this connection. You are dealing with very low voltage and crimp connectors may introduce too much resistance into the circuit.
5. I recommend OEM sensors.
6. Always apply anti-seize compound to the threads of the new sensor.

For more information on this repair or any others for your vehicle, buy an online subscription to either Alldatadiy.com or eautorepair.net. Click on this link to compare the two services: Compare Alldata and Eautorepair. If you just need information for a single repair and want to save money, eautorepair offers a lower price 1-week subscription for only $11.99. Or, if you’ll be working on this vehicle in the future, you can buy a 1-year subscription (Alldatadiy.com for $26.99, or eautorepair.net $29.99)

© 2007 Rick Muscoplat

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