O2 Sensor Fluctuating in Park and High Trans Temps

[Robb235]

StackPath

"The air-fuel sensor is going to have little humps when graphed. The post-cat oxygen sensor paired with it should not oscillate, but instead stay pretty steady somewhere between 500 to 700 mV."

Which sensor description matches the voltage readings shown by OP? Seems like the sensor 1 shown by OP is pretty steady which matches the post catalytic converter voltage readings described here which match what I see when viewing data with pirated techstream. Also, the post catalytic converter oxygen sensor is the main contributing factor in determined catalytic converter efficiency for P0420. I don't know the range because my 2000 FSM link isn't working but I would suspect IF the voltage swings shown by sensor 2 in OP's data were actually for sensor 2 then a P0420 would set.

You sure do have selective reading...

"Before the catalytic converter, the oxygen sensor will zigzag up and down. To the contrary, the air-fuel sensor will be at a stable voltage. The post-cat oxygen sensor will be a straight line if the catalytic converter is good in most cases."

OP has an AFR sensor in front, so like your source stated, the reported voltage should be a stable voltage. In OP's case, his rear O2 sensor is oscillating, so guess what the conclusion is? He already has a P0420 CEL, so we can conclude that most likely his catalytic converter is operating below the 95% efficiency threshold that the ECU looks for. Not that his scantool is bunk.

 

[Bad Luck]

You sure do have selective reading...

"Before the catalytic converter, the oxygen sensor will zigzag up and down. To the contrary, the air-fuel sensor will be at a stable voltage. The post-cat oxygen sensor will be a straight line if the catalytic converter is good in most cases."

OP has an AFR sensor in front, so like your source stated, the reported voltage should be a stable voltage. In OP's case, his rear O2 sensor is oscillating, so guess what the conclusion is? He already has a P0420 CEL, so we can conclude that most likely his catalytic converter is operating below the 95% efficiency threshold that the ECU looks for. Not that his scantool is bunk.

You're right. I forgot OP had P0420 already. I don't have any problem admitting when I'm wrong. I still can't get on board with calling it a "wide band" though.

 

[phattyduck]

What would you call an air/fuel sensor with only 60% of the sensing range of most wide band air/fuel sensors? I wouldn't consider that a wide band, but clearly you do.

You have your opinion and I have mine in that respect. I have never seen my rear oxygen sensor readings fluctuate the way that OP shows. I have California emissions as well.

Yes, it is still a wideband sensor. A narrow band sensor has about a 0.2 range, and around 30x that is pretty wideband by comparison.

The extra range of a UEGO or LSU sensor is not much compared to the range already provided by the single-cell AFR sensor that the 5VZ uses.

-Charlie