Bank 1 on a V-6 or V-8 engine is the left, or driver's side. Bank 2 is the right side. Upstream oxygen sensors are on the engine side of the catalytic converter, and downstream oxygen sensors are after the converter(s). 1995 and older models only had the upstream sensors.
As the exhaust gas leaves the engine, it passes the oxygen sensor first, then goes through the catalytic converter, then the muffler, then out the tail pipe. During proper operation, the exhaust gas switches between too rich and too lean about two times per second. When it's too lean, there's excess unburned oxygen in that exhaust gas. It gets stored in the catalyst. When the exhaust gas turns too rich, the excess gas mixes with that stored oxygen and is burned. Over time, the fuel-to-air ratio is near perfect, but it's hard for the Engine Computer to know when it's as close to perfect as possible. It can, however detect those lean and rich conditions. That's why they switch continuously between rich and lean.
The exhaust gas should be cleaned up as it passes through the catalytic converter(s). 1996 and newer models have the "OBD2", (on-board diagnostics, version 2) emissions system. Those add the downstream oxygen sensors. As the exhaust gas continues on, it passes by the downstream oxygen sensor(s). If everything is working properly, those will switch between rich and lean perhaps once every minute or two. The Engine Computer makes continuous fine-tuning adjustments to the fuel / air mixture as you're driving, based on the readings from the front, or upstream oxygen sensors. The rear, or downstream sensors, however, have nothing to do with engine performance. They are only monitoring the efficiency of the catalytic converter. On the '95 and older models, that wasn't done. People would find the converter to be plugged, then they'd knock out the catalyst material, and no one would know the difference. You can't do that on the '96 and newer models.
As a catalytic converter begins to lose some of its clean-up capability, some of the exhaust gas will start to look like it did going into the converter, meaning not all of it is getting cleaned up. The downstream oxygen sensor will see that and the switching rate from lean to rich will occur faster; lets say twice per minute. As it gets worse, that switching rate might occur four times per minute. Once the efficiency is totally lost, there's no change taking place in the composition of the exhaust gas. At that time the front and rear oxygen sensors will switch at exactly the same rate. The engine can still be running perfectly fine, and the upstream oxygen sensors are still telling the computer how to fine-tune the fuel / air mixture. It's the downstream sensors that are telling the computer that the converters aren't holding up their end of the job.
Another way to look at this is there's two parts to the emissions story. The first part has to do with the upstream sensors, going back to mid to late '70s. These help the computer run the engine as cleanly as possible. The second part of the story is there will still be some pollution, so the catalytic converters were added to address what little pollution was still being produced. The next step was just to add those downstream oxygen sensors to be sure the catalytic converters were taking care of that little pollution, so now the only thing coming out of the tail pipe is carbon dioxide and water vapor. Part of the system reduces as much as possible the pollution that is developed. The newer part of the system addresses what little pollution is developed anyway.
There is a predetermined point at which the computer sees the switching rate of the downstream oxygen sensor is too fast, then it sets a fault code for "catalytic converter efficiency". Remember, it takes properly-working sensors to detect this problem, then the computer sets that fault code. The fix is to replace the catalytic converter, not the oxygen sensors.
A different group of fault codes has to do with "no activity detected" from one of the sensors. That could be due to a defective sensor, but it is just as likely the cause is a leak in the exhaust system, or even the small port used to introduce outside air for comparison is plugged by sprayed-on rustproofing material. Still other codes refer to things like cut sensor wires, or even internal sensor failures.
It's important to know the exact fault code number. There are dozens of possible fault codes related to the oxygen sensors, and they mean very different things. Many of them require readings from properly-working sensors to be detected. It's also important to understand diagnostic fault codes never say to replace a part or that one is defective. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. When a sensor or other part is referenced in a fault code, it is actually the cause of that code about half of the time. First we have to rule out wiring and connector terminal problems, and mechanical problems associated with that pat.
Here's a list of most of the engine-related diagnostic fault codes:
https://www.2carpros.com/trouble_codes/obd2
Once you know the code number(s), you can look at that list for the definitions, or I can interpret them for you. Here's links to some related articles you might find interesting:
https://www.2carpros.com/articles/how-emission-control-systems-work
https://www.2carpros.com/articles/bad-catalytic-converter-symptoms
Sunday, January 24th, 2021 AT 7:37 PM
