O2 Sensor Question, '96 5.0

[V8Pete]

Got a cel, so checked the codes. Says bank 1, sensor 2 is bad. I assume this is one of the rear 02 sensors. Can someone tell me what the 2 add'l sensors (4 total) are for in an OBDII system, and if I really need to replace the one sensor? I haven't noticed any driveability problems.

 

[Glacier991]

The purpose of the 2nd sensor in each bank is to monitor catalytic converter efficiency. The upstream sensor is used to adjust fuel injection pulse width (eg how much) and varies across a threshold voltage (.445v) in response to that voltage the computer varies the pulse width. The Downstream (post cat sensor) is simply monitored for fluctuation. IF the upstream sensor and fuel injection is working correctly, AND if the CAT is working correctly, there should be little if any variation shown in the downstream sensor voltage. In fact during each drive cycle, I believe that the OBDII system intentioanlly varies the mixture into rich and looks for any downstream changes which might indicate the CAT is not doing it's job. That is the purpose of the 2nd sensor, to monitor CAT efficiency. HTH

 

[V8Pete]

Yes, it helps. Thanks for the info. The code means the sensor's bad, right? I assume it would throw a code for the cat if the cat was bad.

 

[Glacier991]

You didn't share the exact code so I cannot say. Do a search for "Check Engine Light Myths" it will help you. When you ask about CEL codes, it is critcal you give the exact numeric code. People often say Ah it says XXXX (sensor) is bad. No it said P0371 or something like that. After you read the article on CEL Myths you will understand why you cannot just assume it is the sensor in every case. (my guess on a post CAT sensor is that it IS the sensor though - but the code is usually "CAT efficiency")