These are not lean/ rich indicators, not directly, at least. An engine that's having 99.2% of its fuel being subtracted to lean out the mixture probably wouldn't even be running. That would mean the fuel injectors are trying to put 2x the required fuel in the engine. STFT B1S2 means Short Term Fuel Trim Bank 1 Sensor 2, which is the downstream 02 sensor from the catalytic converter. The ECU's basically telling you that the catalytic converter is operating at 99.2% efficiency. Your engine is running well enough.

You want to look at STFT B1, STFT B2, LTFT B1 and LTFT B2 to determine if your engine is running rich or lean. These numbers will fluctuate as the engine runs. This image is my scan tool reading the fuel trims in my 2017 Mustang that you are looking for. 

This question comes up occasionally on the forum.

@yaser provided a link to one of them.

This is really a "no brainer" because MOST cars don't use sensor input from the downstream O2 sensors to control the fuel mixture.

(some cars do. Other cars use the downstream O2 sensor input when the computer determines that the voltage signal it's receiving from the upstream O2 sensor is "implausible")((But in that case you'd be getting a trouble code for the upstream O2 sensor)).

The value range for the downstream O2 sensor FT's is -100% to +99.2%.

So why would there be a B1S2 FT value being displayed on a scanner connected to vehicles that aren't using the downstream O2 sensor input to control fuel trims?  

And why is the maximum value of 99.2% being displayed? 

My understanding is that it has to do with Global OBDll requirements. The scanner requests that data and the computer has to provide a response even if it isn't using the downstream O2 sensors to control fuel. For some reason someone decided that the maximum value of 99.2% being displayed on some scanners would mean that the computer isn't using input from those sensors to control the fuel ratio.

Since this question keeps coming up I thought it might be useful to link a credible source (instead of "some guy on the internet") and guess what? Aside from other forum discussions on this topic between "guys on the internet" there wasn't much out there from anyone that can be considered a credible source.

I was able to find this topic addressed by the Support Staff at BlueDriver. They make scanners so I'd consider them a more credible source than "some guy on the internet (like me)".

https://support.bluedriver.com/support/solutions/articles/43000551799-why-is-fuel-trim-n-a-or-99-2-

https://carkiller.com/scottykilmer/qa/short-term-fuel-trim-b1s2/

I think He is taking about the percentage under the voltage for each o2 sensor not the actually trims itself.. 99.2% is normal it's a default number just to double check I grabbed my scan tool went out check my Mercedes, sisters Cadillac, neighbors Lexus my other neighbors Audi and Chevy they all them where the same 99.2%

I’ve never seen a lean condition of 99.2 %.....

I would love to know if anyone receives answer for this. I am reading same code 07 avalanche 5.3 with the stupid active fuel management. Stuck at 99.2