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16 | February 2016 in reverse. When current is applied to the pump cell, oxygen ions fow out of it and into the sample cell. A controller supplies current to the pump cell, and it's pro- grammed to keep the output of the sample cell at 450 millivolts. This creates a closed- loop control system, and the PCM simply monitors the amount of current supplied to the pump cell to know how much oxygen is in the exhaust. In addition to being extremely fast, this type of sensor can actually measure the amount of oxygen in the exhaust over a very wide range rather than just detect a rich/lean condition. This allows the PCM to control air/fuel ratio over a range from 10.3-to-1 (rich) to about 23-to-1 (lean). In earlier AFR sensors, the sheath around the wiring harness forms a sealed conduit that supplies ambient air to the pump cell. These sensors are vulnerable to contamina- tion, especially if the sheath is damaged (that's one reason we're told not to repair the wiring harness). Newer AFR sensors are confgured differ- ently so the sheath is no longer needed. These sensors are different from each other and require different control circuits, so they're not interchangeable. Sensor heaters Basic four-wire O2 sensors are still used as catalyst monitors and labeled sensor S2 on bank 1 or bank 2 (B1S2 or B2S2). The heater brings the sensor up to operating tempera- ture quickly so it can begin working as soon as possible. The PCM monitors the heater circuits continuously, checking resistance for an open circuit or short to ground. If a problem is detected, the PCM will set a code and turn on the malfunction indi- cator light (MIL), but the sensor can still produce a signal if the exhaust gas keeps it hot enough. AFR sensors might also produce a signal without the heater, but that signal would be completely useless because it's not volt- age... it's a measure of the current sent to the oxygen pump. Temperature affects resistance and resistance affects current fow, so the sensor's fat zirconia strip must be held at a constant temperature to gener- ate an accurate signal. The sensor is heated to about 1,200 degrees F (650 degrees C), double the temperature of a basic heated oxygen sensor. Sensor heaters can draw a lot of cur- rent, so battery voltage is usually supplied directly to the heaters through a relay and a fuse. The heaters' ground circuit is controlled by the PCM. A basic four-wire sensor heater is usually turned on all the time, but the AFR heater's ground circuit is pulse width-modulated to keep the tem- perature constant regardless of exhaust gas temperature. There is no temperature sensor in these heaters, so how does the PCM keep the AFR sensor at a constant temperature? Remem- ber that temperature affects resistance, so the PCM can calculate the sensor's tem- perature by monitoring the resistance in Powerplant Stay current on lambda, titania and AFR sensors A regular oxygen (lambda) sensor signal rises and falls as the A/F ratio toggles between rich (high) and lean (low). A titania sen- sor signal also toggles rich/lean, but lean is high and rich is low. The AFR sensor signal does not toggle between rich and lean because it does not produce a voltage signal. It produces a current sig- nal that the PCM uses to maintain a specifc air/fuel mixture. However, when reporting oxygen sensor readings for on-board diagnostics or for a scan tool, the PCM math- ematically converts that current signal to a voltage signal. At stoichiometry, the oxygen sensor PID will hover near 3.3 volts. If the PCM commands a rich mixture for acceleration, there is very little oxygen in the exhaust and the sen- sor PID will be less than 3.3 volts. A lean mixture will produce a PID above 3.3 volts.