O2 Sensor Heater Circuit 


    Back in 1976, Volvo came out with an O2 sensor on their 240 model. This was long before the term O2 sensor was a part of the everyday vocabulary of a mechanic, but that was about to change. By 1980 the California emission standards were getting stricter and by 1981 the federal emission standards made it mandatory for all cars and trucks to have an O2 sensor.  

    In 1996 the OBD-II regulations came in affect and soon not just one O2 sensor was good enough. Now there were multiple O2 sensors with even more responsibility than just to monitor what was coming out of the tail pipe.

What’s it do?

    An oxygen sensor creates a voltage by means of a chemical reaction between the sensor element and the oxygen in the exhaust passing across that element. Outside air also passes through the sensor and is used as a comparison between the oxygen content in the exhaust and fresh air to determine the voltage output. Believe it or not, the fresh air on many sensors actually travels through the wiring insulation. This is one of the many reasons that you should never use any dielectric grease in or around the connectors or in the case of the older style single wire sensors, (which used a small hole on the outer edge to allow fresh air into it) to be caked with debris such as grease or mud. 

Single wire O2 sensor

    Early O2 sensors consisted of one or two wires. Usually the second wire was just a ground signal. These were unheated sensors that required an external heat source and had to be located close to the engine’s exhaust ports. Which is not the ideal locations to measure the fuel to air ratio. Another limitation of the unheated sensor was that it could take over a minute to reach the operating temperature required to even produce a return signal.

Heated O2 sensor

    The oxygen sensor must be hot (about 600 to 660 degrees or higher) before it will start to generate a voltage signal, so many manufacturers opted for oxygen sensors that have a heating element inside to help reach their operating temperature. The heating element can also prevent the sensor from cooling off too much during prolonged idle conditions or when there isn’t enough heat being generated.
Three and four wire heated oxygen sensors evolved in order to reach operating temperature more rapidly, and the heated O2’s could be placed farther downstream in the exhaust system. The heated O2 sensor could also stay at operating temperature much longer than the unheated sensors. These days, the newer heater elements also require less electrical power than before, and can bring the O2 sensor up to operating temperature, typically in about 10 seconds from dead cold.

O2 Sensor variations

Planar O2 sensor

    Planar sensors use layers of zirconia & alumina bonded together. This technology allows for much faster warm up of the sensor because there is a much lower mass to heat & the heater is in direct contact with the sensing portion. Typical warm up time for planar sensors in is the neighborhood of five to thirty seconds.

Titania O2 sensor

    Titania sensors employ the use of titanium oxide instead of zirconia oxide. Unlike zirconia sensors, they require a base reference voltage to operate and change resistance when the A/F ratio changes. Typically the Titania sensors were used on Nissan mid 80’s to 90’s vehicles but are not used in any modern cars these days. 

Wideband O2 sensor – AFR sensor

    Wideband sensors or air fuel ratio sensors (AFR for short), were introduced in 1994 and are now the industry standard. They eliminate the lean-rich cycling inherent in narrow-band sensors, allowing the control unit to adjust the fuel delivery and ignition timing of the engine much more rapidly.

    The basic older oxygen sensors could only indicate whether the engine is running rich or lean of stoichiometric (lambda = 1), while the AFR sensor can measure the actual amount of oxygen present in the exhaust. This allows the PCM to precisely control the engine’s air/fuel ratio to a much closer tolerance than just correcting for a rich or lean mixture. To keep the AFR sensor working at top proficiency it needs to be kept at a constant temperature so it can measure accurately, which in turn, means the heater circuit has to work efficiently and without fail.

    There is no temperature sensor in AFR sensor for the PCM to detect whether or not the sensor is up to temperature. Instead, the PCM calculates the sensor’s temperature by monitoring the resistance in the sensors heater circuit. Since the heat produced affects the resistance and current flow it’s an easy calculation for the PCM circuits to interpret and determine the actual sensors temperature. 
When a new sensor is first installed, the PCM reads the resistance of the (cold) heater circuit and compares it to the known calibration of that particular type AFR sensor. It will then use that resistance value to begin calculating the sensor’s actual temperature as warms up and under the various engine run condtions.

    These heater’s resistance values can vary 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. It’s another reason why there is no heater resistance specification or table of resistance that you can follow. 
On a lot of newer models, the PCM monitors the voltage continuously and the current monitor can run at least once per a drive cycle, usually after certain drive requirement criteria has been met. Newer Chrysler models on the other hand, their heater monitors run after a complete drive cycle and the coolant temperature has dropped by more than 60 degrees F (16 degrees C) after the engine has shut off.

    Most of the other manufacturers, the voltage monitor in the PCM checks for battery voltage in the heater circuit, and the current monitor checks to see if the current is within a specific range. Depending on the type of car and model, this range can be anywhere from 0.2 amps to 8 amps or more.

Simplified heater circuit test

    A quick little test for those adventurist type mechanics that just can’t break out the old ohm meter or scope but still want to see some results. Try unplugging the sensor and locating the two heater leads. Use an old reliable test light across the two heater circuit wires leading back to the PCM. Make sure the service light is not on or that any O2 codes are stored (clear them if needed). Then, turn the key to the ON position. If the PCM wiring and circuits are good working order, the test light should light up or on some vehicles it may pulse on and off. This will confirm that the PCM is attempting to turn on the heater circuit and all the wiring to and from the PCM are in working order.

    In short, the heater circuit which was originally designed to bring the sensor up to operating temperature has now taken on an even more important role. I wouldn’t be surprised if newer designs will be forth coming with even more changes on the way. Change is just a part of the job even with the O2 sensors.