Powerplant
Oxygen sensor heaters
How do you know if that heater fault code is real? By Jacques Gordon Jacques Gordon has worked in the automotive 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.
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 hen the check engine light is on Nernst cell is a semi-permeable wall made and the scan tool displays oxygen of a material that conducts ions. Electrical sensor codes, you already suspect contacts are attached to each side of the that the real problem might be something wall. When there are different concentraother than the oxygen sensor. When the tions of the same gas on either side of the codes indicate a problem with the oxygen wall, a voltage is generated. sensor heater, that narrows the possibilities In a standard O2 sensor, the cell wall is a quite a bit. However, if it’s Sensor 1, even if thin wafer of zirconia that reacts to different concentrations of oxygen in an engine’s you determine the circuits and power supply are good, simply installing a new sensor hot exhaust stream (660 degrees Fahrenheit isn’t a complete fx because the powertrain or 350 degrees Celsius). When there is a high concentration of oxygen on one side control module (PCM) won’t automatically work with the new sensor heater. They must of the wafer and a low concentration on the other, the oxygen on the “high” side will be introduced to each other frst. cause ions to fow through the wafer to the The air/fuel ratio sensor (AFR), also called a wide-band oxygen sensor, was frst “low-oxygen” side. The fow of ions creates a voltage that’s picked up by the electrodes introduced about 15 years ago, and since about 2005 it’s Sensor 1 in just about every attached to each side of the wafer. This type of sensor can generate about 1 volt, engine. An oxygen sensor only indicates and it’s completely passive. whether the engine is running rich or lean The sensor’s zirconia wafer is shaped of stoichiometric (lambda = 1), while the like a thimble, and the outside wall is AFR sensor actually measures the amount exposed to exhaust gas while the inside of oxygen in the exhaust. This allows the wall is exposed to ambient air. One elecPCM to precisely control the engine’s air/ fuel ratio rather than just correct for a rich trode is attached to the sensor body so it’s grounded at the exhaust pipe. That means or lean mixture. The AFR sensor is a very that only one wire is needed to send the sophisticated measuring device that must voltage signal to the PCM. A hollow sheath be kept at a constant temperature to meaaround that wire provides outside reference sure accurately, so the heater is critical to air to the inside of the thimble. its operation.
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14 | February 2016
