oxygen Sensor Information by Trevor Trumbo 

There has been much debate about the role of the oxygen sensor and whether or not we need one on our bikes. This article will explore, at a glance, what an oxygen sensor does and how to test it.

An oxygen sensor (O2 sensor) is the primary measurement device for the fuel control computer in your car or motorcycle to know if the engine is too rich or too lean. The oxygen sensor is active anytime it is hot enough, but the computer only uses this information in the closed loop mode. Closed loop is the operating mode where all engine control sensors, including the oxygen sensor, are used to get best fuel economy, lowest emissions, and good power.

An oxygen sensor works because it is a chemical generator. It is constantly making a comparison between the oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no oxygen in the exhaust manifold, a voltage is generated. The output of an oxygen sensor is usually between 0 and 1.1 volts. All spark combustion engines need the proper air fuel ratio to operate correctly. For gasoline, this is 14.7 parts of air to one part of fuel. When the engine has more fuel than needed, all available oxygen is consumed in the cylinder and gasses leaving through the exhaust contain almost no oxygen. This sends out a voltage greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts. Usually the output range seen seen is 0.2 to 0.7 volts. The sensor does not begin to generate it's full output until it reaches about 600 degrees F. Prior to this time the sensor is not conductive. It is as if the circuit between the sensor and computer is not complete. The mid point is about 0.45 volts. This is neither rich nor lean. A fully warm O2 sensor will not spend any time at 0.45 volts. In many cars, the computer sends out a bias voltage of 0.45 through the O2 sensor wire. If the sensor is not warm, or if the circuit is not complete, the computer picks up a steady 0.45 volts. Since the computer knows this is an "illegal" value, it judges the sensor to not be ready. It remains in open loop operation, and uses all sensors except the O2 to determine fuel delivery. I cannot say with any certainty that this is how our Super X computer operates, but I would assume it follows this course of action as well. Any time an engine is operated in open loop, it runs somewhat rich and makes more exhaust emissions. This translates into lost power, poor fuel economy and air pollution. The O2 sensor is constantly in a state of transition between high and low voltage. Manufacturers call this crossing of the 0.45 volt mark "O2 cross counts". The higher the number of O2 cross counts, the better the sensor and other parts of the computer control system are working. It is important to remember that the O2 sensor is comparing the amount of oxygen inside and outside the engine. If the outside of the sensor should become blocked, or coated with oil, sound insulation, undercoating or antifreeze, (among other things), this comparison is not possible.

According to the Bosch website, our oxygen sensor should be replaced every 12,000 miles for optimum performance. One piece of advice here. If you are going to replace your oxygen sensor, make sure you use an anti-seize compound that is made for oxygen sensors! Your sensor will be instantly ruined if you use any other silicone gasket sealer, including valve covers, oil pan, or nearly any other gasket or seal that controls engine oil. Leaded gas will destroy an oxygen sensor. If your engine is running rich for an extended period of time, the sensor can become plugged up or even destroyed. So the best advice is check your O2 sensor at the specified intervals and replace when needed to keep your engine running properly.

You might be wondering if or how you can test your O2 sensor. It's a fairly simple process if you take your time to do it right. First, the engine must first be fully warm.  Next, attach the positive lead of a high impedance DC voltmeter to the oxygen sensor output wire. This wire should remain attached to the engine computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket. Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do. Check the specs for your meter to find out. Set your meter to look for 1 volt DC. To test for a signal, you should run the engine above 2000 rpm for two minutes to warm the O2 sensor and try to get into closed loop. Closed loop operation is indicated by the sensor showing several cross counts per second. It may help to rev the engine between idle and about 3000 rpm several times. The computer recognizes the sensor as hot and active once there are several cross counts. You are looking for voltage to go above and below 0.45 volts. If you see less than 0.2 and more than 0.7 volts and the value changes rapidly, you are through and your sensor is good. If not, is it steady high (> 0.45), near 0.45, or steady low (< 0.45)? If the voltage is near the middle, you may not be hot yet. Run the engine above 2000 rpm again. CAUTION!! If the reading is steady low, add richness by adding some propane through the air intake. Be very careful if you work with any extra gasoline as you can easily be burned or have an explosion. If the voltage now rises above 0.7 to 0.9, and you can change it at will by changing the extra fuel, the O2 sensor is usually good. If the voltage is steady high, try to create a vacuum leak. If this drives the voltage to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good. If you are not able to make a change either way, stop the engine, unhook the sensor wire from the computer harness, and reattach your voltmeter to the sensor output wire. Repeat the rich and lean steps. If you can't get the sensor voltage to change, and you have a good sensor and ground connection, try heating it once more. Repeat the rich and lean steps. If still no voltage or fixed voltage, you have a bad sensor. If you are not getting a voltage and the bike has been running rich lately, the sensor may be carbon fouled. If the sensor is carbon fouled, fix the cause of the rich mixture and retest. If you don't, the new sensor will fail.

For you car buffs, you can check the sensor the same way except that many late model cars use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire. When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check your connections. Next, check your leads to make sure you won't wrap up any wires in the belts, etc. then start the engine. Then use the same procedure detailed above. You can create an air leak (for lean) by pulling the PCV valve out of it's hose or unhook the power brake vacuum supply hose.

As you can see, the oxygen sensor is an integral part of the engine management system. It provides important information to the engine computer and allows us to get more power, better fuel mileage, and lower emissions. 

Ride Safe, Trevor Trumbo