Rick's Free Auto Repair Advice

Test and replace an O2 Sensor

How to replace an O2 sensor

Most people automatically assume an O2 sensor trouble code means they should replace the oxygen sensor. They immediately want to know how to replace an oxygen sensor. However, if you jump to that conclusion without actually testing the sensor, you could be wasting your money. Take a minute to understand how an O2 sensor works and how they go bad.

What causes an O2 sensor trouble code?

Oxygen sensor trouble codes start with P0130 and go up to P0167. Newer style air/fuel ratio sensors (not covered in this article) may have enhanced or manufacturer specific codes. For more information on oxygen sensor trouble codes, go to this page:

P-series trouble code definitions

P-series trouble code fixes

Getting the right air to fuel mixture for good combustion can get tricky. It’s a complicated process that depends on many variables: the outside air temperature, barometric pressure, engine temperature, whether you’re accelerating or decelerating, and the condition of the engine. Based on those variables, the computer calculates and air/fuel mixture. Then it takes readings from the oxygen sensor to determine how well it did in computing the air/fuel mixture.

If there’s too much oxygen left in the exhaust, one possibility is that the air/fuel didn’t ignite properly due to a bad spark plug causing a misfire. Or, the fire could have gone out early due to too little fuel.  How does the computer determine which problem caused the high oxygen reading?

How does an O2 sensor work?

Since you’re constantly adjusting the gas pedal and the vehicle is constantly encountering changing pavement and slope conditions, the computer must constantly change air/fuel mixture and then check to see how well it calculated the mix.

So the computer is constantly over-shooting and undershooting the ideal mixture with the goal of narrowing the window with each try.  What the computer sees from the oxygen sensor is a constant flip-flopping between rich and lean readings. A properly operating O2 sensor reports a reading 5-7 times per SECOND.

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

A traditional oxygen sensor works like a miniature electrical generator. Many of the older sensors are made out from 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.

What damages an O2 sensor?

Because O2 sensor 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. O2+sensor+coolant+contamination+with+captionWorn rings can coat the bulb with oily soot. That’s a reason to replace an O2 sensor.

If you have any of these engine problems, or use any non-sensor-safe silicone materials in the engine compartment and then get an O2 sensor trouble code, you can bet that you’ve damaged your sensor. That’s another reason to replace an oxygen sensor.

Beyond those types of catastrophic failures, oxygen sensors last a fairly long time—between 60-100,000 miles. But they O2+sensor+oil+fouled+with+captiondo 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. Replace an oxygen sensor when it gets “lazy.”

How to diagnose an O2 sensor?

Many auto parts store will read trouble codes for free. But the biggest mistake do-it-yourselfers (DIY’ers) make when they get anO2 sensor related trouble code is to assume code means they have a bad oxygen sensor. They may have a bad oxygen sensor, 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 O2+sensor+silicone+contamination+with+captiondiagram 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” O2 sensor 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” O2 sensor 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.

How to test an O2 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 an O2 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. Remove the old oxygen sensor and replace an oxygen sensor with the same type and style used by the car maker.

6. I recommend OEM sensors.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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