Fuel trim sometimes close to 0, sometimes goes to +-20.

 

Same situation here. Thought all i had was a lazy sensor. Bank 1 Sensor 1 switched much slower then Bank 2 Sensor 1. Changed the sensor out and it appears to be switching much faster but little improvement on the fuel efficiency. I had gotten a high idle SES code a while back. Im thinking either a vacuum or exhaust manifold leak. Just been too damn cold for me to get the motivation to find the leak.

 

According to manual voltage should be around 1.5V. Fancy Snap-on scanner ($8K) shows 1.5 rear and about 1.3V front.
My cheap Actron scanner ($200) shows .3 and .5, probably doesn't read right. Anyway voltage has to be about the same. I have exhaust leak, I'll deal with it today.
MAF is probably bad, when engine is not running it shows some air coming in.

 

I know what ya mean about different readings from different tools. I have been using a bluetooth OBD2 dongle and some apps on my phone. I noticed the exact differences between torque and car gauge pro with the voltages. The software is reading a different PID im sure of.

Car gauge pro can do some weird useless to me ****. For example, shut down an individual cylinder, or active the abs pump. But is also has options that i will use once i find my problem. Such as, idle air relearn, reset the self learning control, and adjust the timing.

Maybe you could give me some insight. My first sign of an issue was about a year and a half ago. SES light came on with a P0507 code. Pretty generic code saying that the idle rpm was high. I thought the idle rpm was fine and had not changed. Cleared the code and it never came back. The car continued to run fine with a good mpg. I got about 27mpg highway and 19mpg highway with a combined of 22-25mpg. At this time the mpg was fine.

Then a year ago the SES light lit up again with a P0420 code. Dropped it off at nissan with ~79500 miles on the odometer with the SES light on for a possible warranty repair. Warranty ran out at 80000 miles Got a call the next day saying there was no SES light or any pending codes. When i picked the car up i immediately noticed a difference in the idle. Before it idled at about 750rpm and now it idles at about 500rpm. I could feel the engine running where as i could not before.

Then about 5 months ago my mpg started to drop but the car continues to drive just fine. Currently i get about 22mpg highway and 15mpg city with a combined 19mpg. This is driving like im trying to have the smallest carbon footprint possible. Everyone passes me haste fully at the stop lights. But i have no loss of power that i can tell. I get on it every once in a while and will do a 45-90 pull here and there to satisfy my craving to hear the RC2 intake growl.

About 3 months ago the SES light came back on with a P0420 code. Looked at the log of sensor voltages and came to the determination that bank 1 sensor 1 was lazy and not switching as quickly as Bank 2 sensor 1. Replaced the sensor and PCV valve then cleared the code. The fuel mileage stayed about the same. Figured there was something else wrong. P0420 came back after about 1000 miles.

The bank 1 running rich all the time destroyed the O2 sensor and possibly or is destroying the pre-cat. Going to get some vacuum line from the parts house this weekend and replace every line. Also going to spray some carb cleaner around the throttle body and collector manifolds to check them for leaks. Only other place excess air could be getting in and causing the rich condition is the exhaust manifold. Not looking forward to replacing the pre-cat or manifold gasket. Can not hear any vacuum or exhaust leaks at this time.

BTW im not a mechanic in any way. Just a telecommunications service tech with an above average set of tools that im not afraid to take **** apart with.