O2 sensor

Why do you think that?

The downstream 02 sensor, unlike the upstream 02 sensor, has no ability to adjust the fuel mixture or for that matter, anything about how the engine runs. Its sole function is to check the cat for proper function and display a Check Engine light if it detects that the cat has gone bad.

Jerry

Actually, the PCM will use downstream O2 input to "fine tune" fuel trim in addition to monitoring the cat.

The OP didn't say what engine he had, but there's a TSB for 4.0L engines concerning decarbonizing the combustion chamber and revised valve springs.

Why do I seem to recall that my '97 FSM says the downstream sensor is only there to monitor the cat, and that the upstream sensor checks and tells the PCM how to adjust the injector's pulse duration for proper fuel mixture?

Jerry

Yes, the primary reason for the downstream O2 is to monitor cat converter efficiency. That was mandated by the OBD II standard. Since they had to install the 2nd O2 anyway, I guess they thought they'd put it to other uses also, and they did.

The downstream O2 sensors spend most of their time NOT monitoring the cat. The cat monitor isn't a continuously running monitor. Once certain enabling conditions are met, the cat monitor will run, and takes probably less than 30 seconds to complete, and will only run once per trip (key cycle). That leaves a whole lot of driving time to do "nothing". If they can use that time to improve fuel economy or emissions they will, and do.

Can't tell ya why you don't remember seeing that, or why they wouldn't tell ya that in the FSM, but they'd also have you believe that when you look at your oil pressure gauge, you're seeing actual oil pressure.

From the FSM:

The catalyst monitor uses dual oxygen sensors (O2S's) to monitor the efficiency of the converter. The dual O2S's sensor strategy is based on the fact that as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The upstream O2S is used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic converter. The PCM calculates the A/F mixture from the output of the O2S. A low voltage indicates high oxygen content (lean mixture). A high voltage indicates a low content of oxygen (rich mixture). When the upstream O2S detects a lean condition, there is an abundance of oxygen in the exhaust gas. A functioning converter would store this oxygen so it can use it for the oxidation of HC and CO. As the converter absorbs the oxygen, there will be a lack of oxygen downstream of the converter. The output of the downstream O2S will indicate limited activity in this condition. As the converter loses the ability to store oxygen, the condition can be detected from the behavior of the downstream O2S. When the efficiency drops, no chemical reaction takes place. This means the concentration of oxygen will be the same downstream as upstream. The output voltage of the downstream O2S copies the voltage of the upstream sensor. The only difference is a time lag (seen by the PCM) between the switching of the O2S's. To monitor the system, the number of lean-to-rich switches of upstream and downstream O2S's is counted. The ratio of downstream switches to upstream switches is used to determine whether the catalyst is operating properly. An effective catalyst will have fewer downstream switches than it has upstream switches i.e., a ratio closer to zero. For a totally ineffective catalyst, this ratio will be one-toone, indicating that no oxidation occurs in the device. The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over the legal limit, the MIL will be illuminated.