Bank 1 Sensor 2 Fuel Trim?

Hey all,

Got a 96 2.7L Tacoma with 300k+ on it.

Been watching my OBD live data lately as I wait for my recently ordered new Cat (and exhaust) to fix a P0420 code, and I couldn't help but notice... my Bank 1 Sensor 1 fuel trim is pretty normal short-term and long-term, but my Bank 1 Sensor 2 fuel trim is up at 96.2% and doesn't move. Both O2 sensors are alive and reading similar peaks and valleys on the voltage monitoring.

Does the 3RZ-FE not use Bank 1 Sensor 2 (downstream O2 sensor IIRC) data for fuel trim, or is there another issue I need to uncover? Or is this just normal blown-out cat behavior?

Thanks!
Mike

 

If I understand the O2 monitoring system on these trucks correctly (those who know something I don't please chime in). Bank 1, Sensor 1 (upstream sensor) is used to analyse AFR and adjust fuel trim and mixture accordingly. Bank 1, Sensor 2 (downstream sensor) is used to analyse the efficiency of the catalyst by comparing to Sensor 1. Sensor 2 should read relatively flat if the catalyst is functioning properly and would show similar peaks and valleys to Sensor 1 if the catalyst has failed.

There is only one function of this system that I don't completely understand and that is the calibration function. Apparently Sensor 1 sometimes undergoes a calibration by doing some sort of comparison with Sensor 2. I don't know the circumstances that this occurs under or if it is relevant. All I know is people with O2 simulators (used when one removes their catalyst and wants to avoid a check engine light) have reported issues with leaving it plugged in full time because of that calibration issue. Technically the only thing the computer uses Sensor 2 during normal operation for is checking the O2 levels exiting the catalyst (working cat would have flat output, no big peaks or valleys) but during this calibration I believe it uses the AFR function of the second sensor as well to set a base level average for Sensor 1.

Because these sensors are the same part number they can perform the same functions so you could switch them around for fun if you want and see what it does to trims and voltage.