Hey everybody I was hoping to get some advice on a problem that has recently come up. So I bought an OBD2 Scanner to see why ive been getting lower MPG than usual. And one thing that caught my attention was my short term fuel trim for bank 1 sensor 2. It’s running lean with a 99.2%. How could I fix this? I was originally thinking of just replacing my oxygen sensor, but idk if it could also be a vacuum leak? Btw I have a 2017 Nissan Altima SV if that helps.
Don't pay any attention to that. Your scanner is requesting data in Global OBDll which the computer isn't able to provide so it's defaulting to the maximum value of 99.2%.
You want to look at the Voltage values for that downstream O2 sensor to determine if it's functioning correctly (and/or if your catalytic converter is functioning correctly).
You have a 4 cylinder engine (so it has only one Cylinder Bank)
You want to look at STFT Bank 1
LTFT Bank 1
The Upstream O2 sensor Voltage (which in your case is an Air-Fuel Ratio sensor) ((that "voltage value" being displayed is actually a "conversion" but that's a whole other discussion)) but it can still be useful in Global.
and the Downstream O2 sensor Voltage.
Even inexpensive scanners these days can graph these parameters and it's best to observe them displayed as graphs.
The nice part about doing it that way is that you can see if the sensors are responding to rich and lean conditions (which you can create).
For example, while observing the graphs, mash down on the accelerator pedal and quickly release it.
If the sensors are working correctly, you'll see both sensor graph voltages spike Rich (when you mash down on the pedal and spike down Lean when you release it and then very quickly return to normal. (The downstream will slightly lag behind the upstream).
Also, Nissan displays Fuel Correction as (A/F) ALPHA so look at that.
Alpha = 100 (No air/fuel ratio correction is needed)
Alpha > 100 (PCM is adding fuel because the engine is running lean)
Alpha < 100 (PCM is subtracting fuel because the engine is running rich)
Your complaint is "lower MPG".
Look at your Fuel SYS (status) parameter. Maybe record it as you drive around. Make sure the fuel system is staying in Closed Loop and not switching back and forth between Open Loop and Closed Loop. (Open Loop is going to burn more fuel)
Then look at other things like your MAF sensor data. Make sure when KEY ON (engine not running) that your MAF sensor isn't reporting airflow when the engine isn't running. Then start the Altima and check that the MAF sensor is showing around 2.5g/s at idle (because you have a 2.5L engine).
Look at the ECT data. Make sure the ECT sensor isn't telling the computer that the engine is cold when it's warmed up.
Of course check the obvious stuff like making sure the air filter is clean.
