Magazine for the auto service professional
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18 | February 2016 the heater circuit. General Motors (GM) calls this Resistance Calculated Oxygen Sensor Heater Temperature (RCOHT). The same principal is used to measure coolant temperature, but in this application the measurements must be extremely fast and infnitely more precise. The heater circuit has a calibration resis- tor built into the wiring harness or sensor connector. When the sensor is frst installed and connected, the PCM is commanded to read the resistance in the (cold) heater circuit to know the calibration of that resistor. It will then use that resistance value to calculate the sensor's actual temperature under all conditions. Even with a calibration resistor, the heat- er's resistance varies by a few hundredths of an ohm from one AFR sensor to another. That's why the PCM and the AFR sensor heater must be calibrated to each other. That's also why there is no heater resistance specifcation, so measuring heater resis- tance is not a good way to confrm a good/ bad oxygen sensor heater. In addition, that's the other reason we're told not to repair an AFR sensor's wiring harness; the resistance might change. When replacing an air/fuel ratio sensor, you'll need a scan tool capable of commanding the PCM to relearn the sensor heater's resistance. On some models, simply clearing the trouble codes and turning off the MIL will do the job. Some scan tools won't enter the clear-code mode if no codes exist. In that case, just turn on the ignition and disconnect any convenient sensor to create a code. On some models the relearn must be done with the sensor at ambient temperature. Powerplant Courtesy of Snap-on Solus Ultra Courtesy of Robert Bosch Corp. On this 2013 Honda Acura 2.4L engine with the scan tool reading enhanced (OEM) data, the air/fuel sensor signal is reported as both voltage and current. It is interesting to note that they don't cycle up and down when the rpm is steady.