O2 sensor readings

[FSFRS]

Hey guys.

Ever since I’ve got a p0420 (and installed a 10k miles used oem header) I’ve been monitoring my voltage for the O2 sensors. Installing the new oem header fixed the p0420.

Is the precat sensor supposed to read around 2v? Am I supposed to be reading the lambda PIDs? To my understanding the upstream sensor is supposed to oscillate, correct? It’s just staying steady at 2.2ish volts for o2s11(v).

Also, the downstream sensor, o2s12(v) drops to .1 or so when I’m either cruising (decel) and sometimes when I’m at idle. Is this normal? Or indicative if a leak? It’s around .600-800v otherwise.

My long term fuel trim hovers around 2.

Normal operation? Or am I looking at the wrong data?

It’s not throwing any codes right now, and hasn’t since I put in the new oem header.

[cmiovino]

There are two O2 sensors. The first is "pre-cat" and is really "air-fuel ratio" sensor, with the main job of monitoring the exhaust and adjusting fueling. It doesn't throw the P0420 code - but it will trigger other codes of your mixture is too rich/lean, etc.


The post-cat sensor (aka "downstream") is in the front pipe behind the secondary cat there. It's purpose is to strictly look to see if the cat is doing what it's supposed to be doing and a main job emissions (aka throwing the P0420 when it's out of tolerance). It does not do anything with fueling.


If installing a new OEM header fixed the P0420, the cat in the header might have been bad, but I don't see how if it was only 10k miles old. Either way, the rear O2 sensor didn't like what the cat was doing.


I don't know a ton about voltages, so I'll refrain from speaking on that. As far as fuel trims however, there are long and short term fuel trims. Short ones modify the long term values in real time. Long term fuel trims are learned values that allow the car to run in different fuel (by state, octane level [91 vs 93], climates, and even with different mods to some extent. No car is exactly the same. "2" is usually a percentage and ideally should be as close to zero as possible. 2% is totally fine. You just don't want something like 10%+ and at some point it will hit a tolerance threshold and throw either a too rich or too lean CEL code.


Looking at long term fuel trims can be helpful in finding issues like vacuum leaks and such, but if you're at 2% and it's all running good with no CELs, you're good to go. If you have a stock car, you can't even see these ECU values.

[p1l0t]

I feel like it's supposed to be 0-1v but really the way to test it is to measure the resistance (Ohm Ω) at different temperatures.

These guys that sell the kits that fake them online for track use claim they vary the voltage or whatever but I think I remember just slapping a 9 ohm resistor on my STi once and having that work too. (Although it was pro-tuned and they knew about it not sure how a stock ECU on a newer BRZ would react to that)
https://www.magnumtuning.com/en/deta...tor/subaru/brz

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[FSFRS]

Originally Posted by cmiovino

There are two O2 sensors. The first is "pre-cat" and is really "air-fuel ratio" sensor, with the main job of monitoring the exhaust and adjusting fueling. It doesn't throw the P0420 code - but it will trigger other codes of your mixture is too rich/lean, etc.


The post-cat sensor (aka "downstream") is in the front pipe behind the secondary cat there. It's purpose is to strictly look to see if the cat is doing what it's supposed to be doing and a main job emissions (aka throwing the P0420 when it's out of tolerance). It does not do anything with fueling.


If installing a new OEM header fixed the P0420, the cat in the header might have been bad, but I don't see how if it was only 10k miles old. Either way, the rear O2 sensor didn't like what the cat was doing.



I don't know a ton about voltages, so I'll refrain from speaking on that. As far as fuel trims however, there are long and short term fuel trims. Short ones modify the long term values in real time. Long term fuel trims are learned values that allow the car to run in different fuel (by state, octane level [91 vs 93], climates, and even with different mods to some extent. No car is exactly the same. "2" is usually a percentage and ideally should be as close to zero as possible. 2% is totally fine. You just don't want something like 10%+ and at some point it will hit a tolerance threshold and throw either a too rich or too lean CEL code.


Looking at long term fuel trims can be helpful in finding issues like vacuum leaks and such, but if you're at 2% and it's all running good with no CELs, you're good to go. If you have a stock car, you can't even see these ECU values.

I actually don't know the mileage of my original header. The old owner ran catleess UEL, and only had it since 102k miles. I bought it at 122k miles, and he put on either a junkyard header cause he sold the original one, or it's the original header. Whatever it is, it didn't even have the heatshields or anything, nor did it have original hardware, and he double gasketed the header to over pipe connection. (which may also have contributed to the p0420). Old owner told me he put the car back to stock tune so I can only assume it's stock. Everything else on the car is stock aside from the catback.

The honeycomb looked fine when I pulled it off. The "new to my car" oem header I bought was used for 10k miles by someone else who went UEL Catless too.

But yeah I think I'm worrying about it too much at this point, and the CEL is already gone, lol. It's been driving me crazy but this is what I signed up for buying a high mileage 2013, I guess.

I've learned a lot just from this code alone. I saw all the data with a $40 scan tool from amazon.

[thomasmryan]

just a casual glance at the price of the sensors, I would make the assumption it is an air/fuel sensor (aka wideband) that puts out a steady voltage.



Regular or typical oxygen sensors used in most vehicles do switch from about 0.4 to 0.9 volts.



subaru used A/F sensors in the 01 RS




your low voltage on the rear sensor may be an exhaust leak or a failed heater in the sensor. A functioning cat will be hotter at the outlet if you have a pyrometer.