So, oxygen sensors and PCM controls - my favorite thing to teach about. I mean that in all seriousness. This just came across on a FB post, and I assisted with this story-book you are about to endeavor into.

Let's start with open and closed loop. Its simple, open loop in basic terms mean the engine is disregarding your oxygen sensors input. It does not care what the oxygen sensor has to say. Closed loop is the opposite, now the PCM cares what the oxygen sensor says and builds what's called, as fuel trims off of that data.

Now, when do you use open loop and closed loop? Well, when using an oxygen sensor, the sensor has a heat-up time and a dead/off time. The heat-up time is the time needed to warm up so it can read correctly. This means, for at least the first 30 seconds and up to a few minutes, the engine is running. The PCM does NOT care what the oxygen sensor is saying, because it's in open loop. The oxygen sensor info could be called “invalid” being that it's not to the correct temp to be accurate yet, anyway.

Also, anytime you are wide open throttle the PCM once again goes into open loop, not caring what the oxygen sensor has to say. That is called our dead/off time. It’s doing this because the sensor is narrowband in its readings. This means that any data it could give is not accurate enough, so the PCM takes control and says "I don’t need you Mr. Oxygen Sensor."

So when does the PCM use the oxygen sensor? During idle, cruise, warm...you know most of the time an OEM car that drives on the road is running. It is there to keep your engine very, very close to stoich AFR for emissions purpose. I mean, that’s what OBD2 is made for after all! Here’s the cool thing, it does a FANTASTIC job at keeping it stoich under these said conditions. So, don’t remove, delete, or alter these fellas, they are there to help you tune, and keep your tune in check.

So, can an oxygen sensor make a car run bad? Well yes, it can! If it is stuck, lean or rich, it will alter fuel trims and make the car run bad. What happens if you unplug an oxygen sensor? In most cases, an unplugged oxygen sensor will still read voltage, right around 450mV.

But how? Well, I’m glad you asked.

The PCM literally gives itself a false reading of 450mV if the oxygen sensor is unplugged. It does this because once again… it wants to be the best on emissions! That 450mV is stoich, so lying to itself helps itself.

Now… let's shake this whole story up. What if you have a car equipped with a wideband oxygen sensor? Late model vehicles are known to have a way better oxygen sensor called a wideband that shakes this whole novel up. Not all manufactures use this sensor, Ford and Dodge being leaders using this. But that’s all for another day.