Scanner info question

Denali n DOO

Original poster
Member
May 22, 2012
5,596
Basically I bought a scanner to be able to read codes and reset the check engine light. Due to the price at the time I ended up with a launch 6830 that also shows live data stream and can record live data. Not that I understand a lot about O2 sensors and fuel trims and stuff. I noticed on a previous drive that when the temp hit about 184* it went from what I think was a closed loop "CL,USING HO2S" to open loop. On a drive yesterday with it hooked up I noticed even after it was warmed up to 192* it stayed in what I think is called closed loop. I also noticed the O2S B2 S2 stays at .7v, that didn't look normal as it wasn't changing and I think it was before. Any idea what I might be looking at. My guess is maybe nothing since I'm not throwing any codes but maybe a sign of something starting? Thermostat sticky?

Any thoughts?
 

Chickenhawk

Member
Dec 6, 2011
779
Just at a quick glance, your temps look low.

What does your coolant temp gauge read when warm? Straight up or a tick or two to the left of straight up?

Also, there are 2 oxygen sensors. Sensor 1 is in front of the cat converter and readings should jump quite rapidly between 200mV and 900mV. Sensor 2 is behind the cat and doesn't control the engine; it only serves to check the efficiency of the cat converter. It should be relatively steady.

Check your STFT and LTFT for signs that your coolant temps are too low (such as a bad coolant temp sensor or a sticky thermostat.) Short term fuel trim (STFT) should bounce rapidly above and below 0%. You may see readings of anywhere from -10% to +10%. Long term fuel trim (LTFT) should be steady and ideally, it should be close to 0%. If it is above +5% or so, then you are running too rich (possibly because of a low coolant temp.)

A 190 degree or 195 degree thermostat like our trucks have only start opening at 190 to 195 degrees. They should normally run above 200 degrees when warm.

Once you learn what the readings mean, you will find out how useful these scan tools are for diagnostics and how trouble codes almost never tell you what is wrong; they only show symptoms, not causes.

So, let us know your coolant temp when warm, where your coolant temp gauge needle sits when warm, your O2 sensor 1 readings and your LTFT readings.
 

Denali n DOO

Original poster
Member
May 22, 2012
5,596
Chickenhawk said:
Just at a quick glance, your temps look low.

What does your coolant temp gauge read when warm? Straight up or a tick or two to the left of straight up?

Also, there are 2 oxygen sensors. Sensor 1 is in front of the cat converter and readings should jump quite rapidly between 200mV and 900mV. Sensor 2 is behind the cat and doesn't control the engine; it only serves to check the efficiency of the cat converter. It should be relatively steady.

Check your STFT and LTFT for signs that your coolant temps are too low (such as a bad coolant temp sensor or a sticky thermostat.) Short term fuel trim (STFT) should bounce rapidly above and below 0%. You may see readings of anywhere from -10% to +10%. Long term fuel trim (LTFT) should be steady and ideally, it should be close to 0%. If it is above +5% or so, then you are running too rich (possibly because of a low coolant temp.)

A 190 degree or 195 degree thermostat like our trucks have only start opening at 190 to 195 degrees. They should normally run above 200 degrees when warm.

Once you learn what the readings mean, you will find out how useful these scan tools are for diagnostics and how trouble codes almost never tell you what is wrong; they only show symptoms, not causes.

So, let us know your coolant temp when warm, where your coolant temp gauge needle sits when warm, your O2 sensor 1 readings and your LTFT readings.

Thanks Chickenhawk for your reply, I was starting to think I was way out to lunch with my question, I think it stayed in closed loop even after warmed up (temp needle pointing straight up). I did a lil google searching for the terms used and such. I'm gonna go through the recorded numbers I have already. Then later today when my wife gets home from work the engine should be good and warm and I'll do a new recording and compare. I'll report back later. Thanks again.
 

Denali n DOO

Original poster
Member
May 22, 2012
5,596
Which O2S number should I be watching, the V rated number or the STF% number?
 

Chickenhawk

Member
Dec 6, 2011
779
You want to be looking at the volts (V) number. It is not important what the number reads; you are looking at how fast it changes. For example, does it change rapidly several times a second, or does it change slowly, several times a minute?

Same with the STFT %. The actual numbers don't mean much; it is how fast they change.

The long term fuel trim (LTFT) % is the number you want to report because it should be relatively steady.

The O2 sensor 2 reading doesn't mean much either. It should just be a relatively steady reading.
 

Denali n DOO

Original poster
Member
May 22, 2012
5,596
Chickenhawk said:
You want to be looking at the volts (V) number. It is not important what the number reads; you are looking at how fast it changes. For example, does it change rapidly several times a second, or does it change slowly, several times a minute?

Same with the STFT %. The actual numbers don't mean much; it is how fast they change.

The long term fuel trim (LTFT) % is the number you want to report because it should be relatively steady.

The O2 sensor 2 reading doesn't mean much either. It should just be a relatively steady reading.

Should it go from closed loop to open loop after it's warmed up? The previous drive it didn't go open loop.

The LTFT was all over -1.55, 2.33, 3.11, 8.58, 9.36, 11.7, 12.48.

I'm going for a drive soon and see what it's doing now.

2 previous maintenance issues with cooling system, coolant reservoir filled with chocolate brown goop that the dealer said was transmission fluid leaking into the coolant. They changed the rad and flushed the system. Oddly enough they never indicated that they had to add transmission fluid. I thought that odd since they said it leaked into the coolant, why wouldn't it need more tranny fluid?

second occurrence the coolant reservoir was filled with chocolate brown goop that the dealer said was engine oil mixed with coolant. They said that I must have added oil directly into the coolant reservoir, other than that they don't know how it got in there. They flushed the system and sent me on my way.

I ended up to change the rad hoses as well because they were getting all spongy. I wonder if the contamination in there has caused damage to the thermostat or made it sticky.

The other thing I'll mention is the last oil change at the dealer was overfilled by about 1/4 on the dipstick. It has leaked a brown fluid, maybe oil, since then. They inspected it a week later and said nothing is leaking. But it still leaks something. Oil about 1/2 way on crosshatches now. Needless to say it's booked at a different dealer for this Thursday to change the fuel level sensor and inspect for the leak.
 

mrphoenix80

Member
Jan 1, 2013
251
Open loop is a cold condition for like 2-5 min and then closed loop unless under heavy load. As I run mine at W.O.T. alot its in open loop alot. So it's not uncommon for it to switch between open and closed loop.
As for the o2 volt readings as you vary the throttle the computer looks at the o2's to adjust fuel mix. 450mv is the ideal mix but it NEVER reads exactly 450. The computer looks at the voltage and says oops to much fuel cut it back oops to little now add more and so on thats why the voltage changes so fast and so much. Also when you open the throttle it adds more air so it shows on the o2 as not enough fuel and then it will correct for it and then when you let off its to much fuel and the computer corrects for that as well. So for diag purposes I look at these voltages to check that the sensor can see the change in air/fuel ratio and that confirms the sensor works.
The STFT% and LTFT% are short term fuel trim and long trem fuel trims. They read the % of fuel added or removed to correct for the air/fuel ratio. Short term will bounce a lot and the long not so much. Long term is what I watch to see the overall fuel system performance. A +-10% reading is good with under 5 ideal, over 25% will set a code. A positive reading is adding fuel to correct for a lean condition and a negitive is removing fuel to correct for a rich condition.

Hope this helps you understand the readings you see on your scan tool.
BTW the thermostant is a 195F so if you dont reach at least that it probably time for a new one.
 

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