Every code has a long list of criteria that must be met before it will set, and one of those things is certain other codes can not already be in memory. The reason is the Engine Computer looks at a lot of different things to compare to know when to set a code. You can get all four if the problems are intermittent and act up at different times, but usually you'll only get one code if they act up at the same time. One thing to be aware of that can be real frustrating for mechanics who have to provide estimates for their customers is in your case with four bad sensors, you likely will not get four diagnostic trouble codes. It is strictly to monitor catalytic converter efficiency. The rear sensor has no affect on engine performance at all. The front one has a very tiny affect on engine performance, in fact it's almost unnoticeable, and it won't even be in the picture yet until it gets up to operating temperature of 600 degrees. That's how the computer determines if the converter is doing its job. When the converter loses its efficiency, no change takes place in the composition of the exhaust gas so the rear switching rate is almost as fast as the front one. Rear one real slow means the catalytic converter is working. Front one real fast means that sensor is working. The secret is the computer looks at those two switching rates and compares them. In fact, it will swing between a little bit rich or lean VERY slowly, as in perhaps once or twice a minute. When the catalytic converter is doing its thing the gas leaving it is neither real rich nor real lean. A constant perfect mixture at all times is ideal with the old carburetors, but with fuel injection we need to see the changes from rich to lean to verify the Engine Computer has control over that mixture. During the rich pulses the unburned fuel is mixed with that oxygen and burned. Over a period of time that averages out to a perfect air / fuel mixture ratio, but during the lean pulses, extra unburned oxygen is stored in the catalyst. If you watch the switching rate with a scanner, the front sensor, (upstream / in front of the catalytic converter) should switch between "rich" and "lean" as much as two or three times per second. To add to the story, you should have a Check Engine light on if any one of those sensors isn't doing its thing. I was surprised to see that they are very inexpensive. Com to verify they are different from the rears, and they are. I know the lefts and rights will be the same but I checked at rockauto.
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