Magazine for the auto service professional
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14 | February 2016 Oxygen sensor heaters How do you know if that heater fault code is real? By Jacques Gordon Jacques Gordon has worked in the automo- tive industry for 40-plus years as a service technician, lab technician, trainer and technical writer. His began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifcations and has participated in ASE test writing workshops. W hen the check engine light is on and the scan tool displays oxygen sensor codes, you already suspect that the real problem might be something other than the oxygen sensor. When the codes indicate a problem with the oxygen sensor heater, that narrows the possibilities quite a bit. However, if it's Sensor 1, even if you determine the circuits and power sup- ply are good, simply installing a new sensor isn't a complete fx because the powertrain control module (PCM) won't automatically work with the new sensor heater. They must be introduced to each other frst. The air/fuel ratio sensor (AFR), also called a wide-band oxygen sensor, was frst introduced about 15 years ago, and since about 2005 it's Sensor 1 in just about every engine. An oxygen sensor only indicates whether the engine is running rich or lean of stoichiometric (lambda = 1), while the AFR sensor actually measures the amount of oxygen in the exhaust. This allows the PCM to precisely control the engine's air/ fuel ratio rather than just correct for a rich or lean mixture. The AFR sensor is a very sophisticated measuring device that must be kept at a constant temperature to mea- sure accurately, so the heater is critical to its operation. To understand the heater in an AFR sensor, it will help to review how an oxygen sensor works. There are three types of oxygen sensors: the passive Nernst cell, the powered titania sensor and the AFR or wide-band oxygen sensor. The basic oxygen sensor is based on the Nernst cell, named for the German physicist who developed the equations that defne how it works. In physical chemistry, a Nernst cell is a semi-permeable wall made of a material that conducts ions. Electrical contacts are attached to each side of the wall. When there are different concentra- tions of the same gas on either side of the wall, a voltage is generated. In a standard O2 sensor, the cell wall is a thin wafer of zirconia that reacts to differ- ent concentrations of oxygen in an engine's hot exhaust stream (660 degrees Fahrenheit or 350 degrees Celsius). When there is a high concentration of oxygen on one side of the wafer and a low concentration on the other, the oxygen on the "high" side will cause ions to fow through the wafer to the "low-oxygen" side. The fow of ions creates a voltage that's picked up by the electrodes attached to each side of the wafer. This type of sensor can generate about 1 volt, and it's completely passive. The sensor's zirconia wafer is shaped like a thimble, and the outside wall is exposed to exhaust gas while the inside wall is exposed to ambient air. One elec- trode is attached to the sensor body so it's grounded at the exhaust pipe. That means that only one wire is needed to send the voltage signal to the PCM. A hollow sheath around that wire provides outside reference air to the inside of the thimble. Powerplant