SERVICE ity the oxygen sensor. high idle between 1200 and 2000 rpm. Every day of its working •Artificially drive the system rich (us- life, it’s exposed to ex- ing propane) and lean (by opening the treme heat and harmful brake booster hose) as you observe the . This humble part oscilloscope voltage reading. It should has a direct effect on ve- peg from .800 to .900 volt (full rich) to hicle emissions, driveability and fuel .100 to .200 volt (full lean). economy, yet few motorists could iden- Minimum and maximum sensor volt- Ptify one in a lineup. Even if you were ages are just part of the picture. How

able to find a motorist who knows what Seyfert Karl Photo: fast these transitions occur is an equally an oxygen sensor is, it’s unlikely he’d be important measure of O2 sensor health. able to recognize the symptoms of a •Internal contamination by silicone Lean-to-rich and rich-to-lean transitions slow, worn-out or completely dead oxy- (used in spray chemicals, materi- should occur in 300mS or less; anything gen sensor. That’s why we can’t count al, etc.). greater may indicate a worn or contami- on him to schedule an appointment be- •The sensor has become coated with nated sensor. An oscilloscope or graph- cause he thinks his needs oxy- other residual chemicals. ing multimeter is essential for this test. gen sensor service. •External contamination by other au- A limited number of are When an O2 sensor fails or is begin- tomotive chemicals (undercoating solu- equipped with a new type of oxygen ning to fail, the vehicle owner may tions, etc.) applied in proximity to the sensor called an air/fuel ratio sensor. come to you stating “I think it needs a O2 sensor. Unlike the toggling voltage produced by tuneup.” There’s a certain logic to this, •Fueling with contaminated or lead- conventional , the air/fuel ratio since the symptoms can be very similar ed , causing lead fouling. sensor produces a linear voltage signal. to what we used to see when ignition Testing methods are also different. This parts were defective or worn. These type of sensor allows more precise mon- symptoms might include: Sensor Inspection & Testing itoring and correction of the fuel mix- •Failing an emissions inspection. The oxygen sensor should be inspected ture than was possible with convention- •Excessive fuel consumption. during every engine performance ser- al O2 sensors, and may receive more •Driveability problems, such as en- vice, or whenever its performance is widespread use in the future. gine surging or hesitation. suspect. A visual inspection alone can’t Regardless of their performance •The Check Engine or oxygen sensor verify the functionality of the oxygen characteristics, all oxygen sensors are light appears (on earlier vehicles). sensor. An oscilloscope test of the sen- now heated. Heating the sensor gives it •The on-board computer stores a sor’s ability to respond to changes in ex- a longer design life, in part because it mixture-related diagnostic trouble code. haust oxygen content, as well as a mea- can be placed further downstream in •Fouling spark plugs. surement of its response time, will pro- the . This protects the Oxygen sensors are not unlimited-life vide a more definitive diagnosis. sensor from the thermal shocks caused components. It’s true that newer de- To test an oxygen sensor’s ability to by engine heating and cooling cycles. signs last longer than their predeces- respond to rich and lean conditions, fol- Heated sensors also reach operating sors, which explains why O2 sensor re- low these steps: temperature quickly and begin pro- placement intervals were often included •Connect the oscilloscope leads to ducing a sensing voltage without hav- in vehicle maintenance schedules. To- the O2 sensor circuit. For vehicles that ing to wait for the heat of the exhaust day, most vehicle manufacturers have use a three- or four-wire heated O2 sen- to warm them. changed that strategy and consider oxy- sor, you may have to bridge the connec- Heated oxygen sensors have smaller gen sensors to be “replace when they tor leads and tap into the signal wire slots in the protective tube at the tip. fail” components. When (or if) an oxy- with an appropriate test probe at the This limits the amount of exhaust that gen sensor fails, there could be several connector plug. reaches the sensing tube, and limits the logical reasons why. These include: •Warm the engine to operating tem- contaminants that can reach the sensor •The unit has exceeded its design-life perature (oxygen sensors work properly and diminish its response. Consequent- expectancy. only when they’re hot—at least 350°F ly, a heated O2 sensor may last 100,000 •The vehicle burns too much oil or 600°C). Run the engine at about miles or more. (causing sensor carbon fouling). 2500 rpm for two or three minutes to Visit www.motor.com to download •An internal engine leak heat up the sensor. a free copy of this article. has contaminated the sensor. •Hold engine speed at a steady,

BY KARL SEYFERT 48 July 2003