P0442 MIL

[Fless]

Interesting subject, and I've been reading up. This article might shed some light on what to expect.

https://www.aa1car.com/library/o2sensor.htm

What I've been reading is that, generally, the mv reading should move around between 100 mv (lean) and 900 mv (rich), but the desired value is around 450 mv. Obviously the sensors are always changing, which is the point of their function.

If you're seeing consistent values lower than 100 mv, I would think that the sensor might be somewhat out of range (weak?) or there might be a problem on that bank. It might be worthwhile to swap the two that are in their respective positions to see if the pattern follows.

 

[wsteele]

Sorry I can't help with that since my downstream O2s are no longer there.

I'm sure you probably know this but the downstream O2s play absolutely zero role in fuel trims.

I frankly have always been a little fuzzy on the role of downstream O2 sensors. Clearly, the ECU monitor for cat function uses it, but beyond that it seems like a function of reporting things after the fact. Maybe their only function is to report when a cat quits being effective?

 

[kbuskill]

I frankly have always been a little fuzzy on the role of downstream O2 sensors. Clearly, the ECU monitor for cat function uses it, but beyond that it seems like a function of reporting things after the fact. Maybe their only function is to report when a cat quits being effective?

That is correct. They are essentially Catalyst monitors and when the cats stop being effective they tell the ECM to throw a code.

Upstream O2s control your fueling.

 

[jj88]

I've got a long term P0442 code on a 2003 blazer... and its NOT the gas cap. 1 point for @PNW VietVet =)

I think its the rubber tubing to the gas tank from the fill port though. Not sure. Need a smoker.

 

[OR VietVet]

I've got a long term P0442 code on a 2003 blazer... and its NOT the gas cap. 1 point for @PNW VietVet =)

I think its the rubber tubing to the gas tank from the fill port though. Not sure. Need a smoker.

That evap system is a source of many complaints. I have recommended before and will do it again, "Replace the whole system along with the cannister." I have seen too many stories here about guys pulling their hair out trying to fix one piece at a time.

 

[OR VietVet]

I frankly have always been a little fuzzy on the role of downstream O2 sensors. Clearly, the ECU monitor for cat function uses it, but beyond that it seems like a function of reporting things after the fact. Maybe their only function is to report when a cat quits being effective?

The fuel/emissions systems do not go from open loop to closed loop without the oxygen sensors and catalytic convertors working properly.

 

[wsteele]

The fuel/emissions systems do no go from open loop to closed loop without the oxygen sensors and catalytic convertors working properly.

It would be interesting to know what those downstream O2 values have to be to trigger closed loop, in post #24, bank 2 sensor 2 doesn’t seem to be functioning yet (engine just warming up), but the system is running in closed loop.

 

[OR VietVet]

For sure, the sensors have to reach a certain temp before functioning properly. Then they have to be able to "switch" fast as they calculate what is being fed to them and then sending out info to ECM so adjustments are made.

 

[wsteele]

For sure, the sensors have to reach a certain temp before functioning properly. Then they have to be able to "switch" fast as they calculate what is being fed to them and then sending out info to ECM so adjustments are made.

Right, but as it pertains to open/closed loop, it didn't seem to matter when I was warming my engine up that Bank 2 Sensor 2 wasn't reporting an accurate number to affect it going into closed loop. It would be interesting to know exactly when the downstream O2 sensors have to be working properly for it to affect open/closed loop.

Reviewing that clip, the Bank 1 Sensor 2 didn't seem to be properly switching yet as well. I wonder how the downstream sensors do affect open/closed loop.

 

[OR VietVet]

It's the front sensors that do most of the work. They are cold started, get a rich fuel mixture and at that point they are not even reading things. They don't read till they heat up to a certain point and then they will feed info to ecm to go from open loop to closed loop. Then the rear sensors have also heated up and they tell the ecm the fuel/air mixture percentage that is coming out of the cats and then the ecm makes adjustments accordingly and then the front sensors deal with whatever fuel mixture ratio there is and again feed info to the ecm. The ecm will make adjustments off of that info and again after input from the rear sensors. I hope I spell this right, the fuel/air mixture the ecm is looking for is 14.7 parts air to one part fuel. That is called
stoichiometric. You want to see the rapid switching with the sensors when you are seeing live stream info. I recently replaced all 4 sensors with GM sensors when my Bank 1 Sensor 2 went bad. Eventually I will do both cats. 135k miles on my rig means some things just need to be replaced to count on the rig.