Take it to the dealership. They have the Consult II computer that might be
able to find more than the Pep Boys shop can. Even then, that's not
infallible.

[I had a problem with a 2002 Sentra where the engine would idle high at
warm-up, then fluctuate up and down at idle--between 500 rpm and 1500 rpm.

Taking it to the dealership and them using a Consult II for diagnosis showed
no codes, and attempts at the dealership doing an air/fuel relearn didn't
work.

So I continued to drive the car in that condition, simply because I wasn't
going to leave it with them if they couldn't find the problem, and the car
was still drivable... the problem was only at idle.

While I was still driving the car, the idle would continue to be high at
warm-up, then it would fluctuate after warm-up, and the transmission would
shift gears almost at redline, at least until the torque converter lockup
conditions were right, and then it would shift normally until the next time
the vehicle was shut off.

Finally, after nearly 500 additional miles of mostly freeway driving--the
computer finally lit the Service Engine Soon light, and shortly afterwards,
the O/D off light flashed a code just before the engine was started. I
returned back to the dealership, and they diagnosed a Throttle Position
Sensor code, but also replaced the ECU since both were still under warranty.

The vehicle has run fine and shifted fine ever since then. No idle issues,
no transmission shifting issues. No additional codes reported.]

Also, it could be that Nissan's on-board diagnostics just take a long time
to throw codes even when there is a problem. In your case, you specify that
codes were found even at Pep Boys, so definitely take it to the dealership.