P0141, P0135, P0147, P0155, P0161, P0167 02 Sensor Heater Circuit Malfunction
The good thing about these codes is that for the most part O2 sensors are fairly universal, sure they are made by a bunch of different manufacturers (Bosch being the first one back in the late 60’s) but the principals of operation are almost identical no matter who makes it. My point in this statement is that it doesn’t matter if you get this code on a Ford or a Hyundai the principals of testing are pretty much the same. This code typically refers to a 4 wire O2 sensor although sometimes you’ll see a 3 wire sensor also. The 5 and 7 wire sensors are actually called AF (Air Fuel) sensors and would have different codes as a result, we’ll cover those in another article. The differences between 3 and 4 wire sensors are what those wires represent and in the case of a 4 wire sensor you typically have 2 wires for the heater, one power and one ground which are usually controlled by the PCM. This is most often a 12v circuit with a heating element in the sensor itself. The other 2 wires are a signal wire and a ground wire. The signal wire is the output signal of the O2 sensor itself and the ground wire is the optional 4th wire. With the 3 wire sensor the O2 signal is grounded through the exhaust system typically. Now that we know what wires we’re dealing with we can do a simple test to confirm that indeed the sensor heater is faulty. A quick test is to just unplug the sensor and do a resistance check on the heater circuit. In my experience the wires for the heater circuit are often the same color, usually 2 white or 2 black and normally right next to each other in the connector. Check the wiring diagram for your application to be sure because you want to get this right. Normally if the heater is bad you’ll see infinite resistance or an open circuit, if you see resistance and the sensor is bad it will probably be very high resistance however if your resistance is low say less than 20 ohms or so then the heater may very well be good. This could mean a couple of things. The first is the sensor is not the problem and you’ll need to check the wiring and the output from the PCM, the second is that you’re on the wrong sensor. This brings me to my next point about this test and that’s locating what sensor is at fault as there are often several O2 sensors in an exhaust system. Figuring out which ‘bank’ is probably going to be the most difficult part so we’ll start with that. Every V engine has a leading bank and a trailing bank as I call them. This is due to the fact that the pistons are staggered on the crank shaft by design and as a result you’ll have one side of the block that’s a little bit farther ahead than the other side. So if you look at the engine block from the front you’ll see that one bank is closer to you than the other, this is bank 1 and the cylinder that’s closest to you is cylinder #1. Now that you’ve worked out what bank is what the next step is to figure out what sensor you have as typically the code is written “P0141 Heater failure (bank 1 sensor 2)”. Sensor #1 is closest to the engine and often right in the exhaust manifold whereas sensor #2 is the next sensor in the exhaust stream of that same bank usually behind or fastened to the catalytic converter. In short the sensors are numbered starting with #1 closest to the engine and the other sensors are downstream of that bank in order, #1, #2, #3…. I know this part can get confusing but once you’ve found what bank you’re looking at I think you’ll have an easier time of it.
Now let’s briefly touch on the operation of a heated O2 sensor or as some call it the ‘Lambda’ sensor. I think a good starting point is to describe how an O2 sensor works. O2 sensors produce their own voltage typically between .1 and 1.0V. Think of an O2 sensor like a potato clock, that’s one of those digital clocks that you can stick a couple of metal strips into a pair of potatoes and the potatoes actually become the power source for the clock. That’s not exactly how it works but the idea is that the clock makes its own power because of the flow of electrons through the potato and the 2 different types of metal that you stick into the potato. With electricity you only need to move electrons and in the case of an O2 sensor it moves electrons based on the difference in the amount of oxygen that its element is exposed to. O2 sensors have an element that is exposed to the air outside the exhaust system AND the air inside the exhaust stream. The air on the outside the exhaust has a pretty steady concentration of oxygen but inside the exhaust the oxygen content can fluctuate a great deal depending on how the engine is running, rich or lean. What the O2 sensor does is make electricity based on the difference of oxygen in the exhaust stream and the oxygen in the outside air. For example say the engine is running rich, there is less oxygen in the exhaust stream and still the same amount of oxygen in the outside air, this difference causes the O2 to produce a high voltage, typically near 1.0v. That 1.0v signal is sent to the PCM and interpreted as a rich condition so the PCM adjusts fuel injector pulse width to lean out the mixture and bring it closer to the optimum fuel mixture for the best fuel economy (this is called stoichiometry). If the exhaust is lean meaning it has MORE oxygen in the exhaust stream the O2 sensor produces a much lower voltage perhaps near .1v which the computer would interpret as a lean condition and it would respond by adjusting injector plus width to add more fuel. This cycle continues over and over with the goal of keeping the reading as close to .5v (stoichiometric ratio) as possible. This state of operation is referred to as “Closed Loop”. Closed loop is achieved when the engine and primary O2 sensor(s) reach operating temperature and the PCM uses the O2 sensor as its main input for fuel mixture adjustments. Once the right coolant temperature is reached the PCM looks to the O2 sensor as its primary input for adjusting the air fuel ratio, in the time before the PCM goes into closed loop operation it ignores the O2 readings and goes on a pre-prescribed fuel mixture that was set at the factory and it does not monitor the output of it’s emissions. The O2 sensors in the catalytic converter or behind the catalytic converter (secondary O2 sensors) are there to monitor catalytic converter efficiency, we’ll cover those in another article but if one of those has a heater failure the testing is exactly the same as for a primary O2 sensor.
So where does the heater come in? Good question. The O2 heater is used to get the O2 sensor to reach operating temperature as quickly as possible as the engine runs most efficient when it’s in closed loop. The O2 sensor doesn’t begin to work efficiently until it reaches about 600ºF (316ºC). If the sensor is below that temp it doesn’t work effectively so manufacturers began to install heaters in them to get them up to temperature more rapidly because the sooner we can get the engine into closed loop the better because remember the engine runs most efficient when it’s in closed loop.
Now let’s touch a little on how the code is set. When you first start the engine cold the PCM tracks the coolant temp as well as the readings from the O2. If the PCM sees that the O2 is not switching back and forth rich lean at a given rate at a given coolant temp it will set the P0141 or similar code. It also monitors the heater circuit for opens and shorts so if it sees one or the other it will also set a code. The thing about these heater codes is that you most likely won’t notice ANY difference in performance, fuel economy may suffer perhaps but not performance. The reason for this is that the O2 is still capable of sending a signal to the PCM it just doesn’t happen as quickly because it won’t reach operating temperature quickly enough or it may cool down causing it to be ‘lazy’ and give inaccurate readings. Here are some videos on how an O2 sensor operates and also one on P0141 diagnosis. Remember just a quick resistance check can tell you if the heater element is bad so it’s a good idea to do this check before you replace the sensor. You cannot typically repair the sensor so replacement is really your only option. If you find the heater to be good (low resistance but not NO resistance) check the wiring for shorts or opens as well as the output voltage from the PCM for the heater. See your service manual for specifics on your vehicle. I hope this information is helpful to you and helps you solve any issues you might run into with O2 heater failure codes.