2 Quick H02S Question

[quote=”dtidman” post=162820][quote=”Getdahorns” post=162817]Hey everyone. I’m working on my uncles ’99 Mazda B2500 and it’s got a rough idle/intermittent stall problem. He told me if he didn’t keep it gassed a little bit it would stall. No codes sadly. Looking at some live data, I saw the 1/2 H02S (downstream) was pegged at 99.2% for the fuel trim and anywhere from .700 to .800 for voltage. The 1/1 H02S (upstream) was reading correctly, fuel trim around 0%, +/-5%. Voltage looked good on it as well, varying as it should. I did a heater circuit test on the suspected problem H02S and it tested around 4.8-5.6 ohms. My 2 questions are:

1. Can a H02S still be bad if it passes a heater circuit resistance test?[/quote]

It could be, but in this case I don’t think in this case it is bad. The upstream will be switching, the downstream O2 is just checking the catalyst efficiency. Check out some of the ETCG or Scanner Danner videos on P0420 or catalytic converters.

[quote=”Getdahorns” post=162817]
2. Do you think this downstream H02S could be the cause of the rough idle/intermittent stalling based on the data I see?[/quote]

I don’t think that is likely, but stranger things have happened.

I would be looking at fuel or air since there are no codes.

Maybe some of the professional techs will have other opinions.[/quote]

Even though its fuel trim and voltage are pegged lean it’s not bad?

[quote=”shaun” post=162828]The upstream sensor is reading normal but your downstream is reading slightly rich above .7 volts? The downstream should be the catalyst one and it should ready steady. And will not have fuel trims. Effincency maybe. Your upstream should be changing.

What’s the LTFT at?[/quote]

Do you want cold or warm values? I can get the car to OT if I hold it at like 2500-3000.

Here are two screenshots of my live data I have. This is during closed loop. The first one is when I started it and didn’t touch the gas or anything, and it surprisingly ran ok. I did a snap throttle to see what it’d do and it started running very rough and tried to stall if I didn’t keep it gassed like before. The MAF doesn’t look like it’s working from the screen shot, but the screen shot was taken right after I let off the throttle, so it went down. It’ll spike to 60 g/s or so as expected. But I noticed it was running fine, and the 1/2 O2 was pegged low, and actually it was varying voltage from .07V to .635V while I let it idle for 5 minutes or so before my first snap throttle. As soon as I snapped the throttle, you can see it went high and stays high even if I let off. These are the readings I recorded from before I touched the throttle since it was idling pretty well:

STFT: 0 – +5%
LTFT: 0%, never changed

1/1 ST: -1% – +4%
1/2 ST: 99.2% the whole time, maxing my scan tool

1/1 Voltage: .1V – .850V
1/2 Voltage: .07V – .635V

Let me know what you guys think, and thanks for all the input so far!

[quote=”dhimm1212″ post=162940]Check the maf, it may be dirty or out of range.
Looking at your fuel trim it screams for fuel, however your data doesn’t show throttle position. Without that I can’t say for sure, but in my experience if the car goes lean under load then the air isn’t being measured accurately.[/quote]

The first screenshot was at idle, and the second one was right after a snap throttle and since it started running poorly, I had to keep it at like 2000 RPM so it wouldn’t die. I can get you a screenshot of the ATP with the other data if you’d like. What PIDs do you want to see with the ATP?

[quote=”shaun” post=162978]It’s compensating for too much air. Your o2 is reading very lean. Make sure your not getting unwanted air. Especially between maf and intake. Vacuum leaks, exhaust leaks before o2’s. Unplug the maf while it’s running and see if you get a change. Is there black smoke out the rear? How’s the egr?[/quote]

I checked the MAF, looked good. Cleaned it anyways. Haven’t tried starting it yet it see if there’s a change. One thing I did notice was something while testing the IAC. I unbolted it and noted the position of the pintle. I turned the key to the on position, and went back to see if it’s changed. Nothing moved. I had someone cycle the key back and forth for me while I watch it and nothing moves. I feel something inside of it buzzing, and then there’s an audible and tangible pop like it slips or something. Since I see no movement in the IAC, do you think it’s fair to say it could be that causing the problem? A new Wells IAC is only $38, but I don’t want to just throw a part at it. I want to be reasonably certain that it’s the issue.