When I talk about non load fuel trim cells, most people assume cell 21 alone is the only cell that is used for lifting off the gas pedal but as I have been mentioning there are are cells that I put in the cell matrix that are also non load cells.

When the engine is required to decelerate, a leaner air/fuel mixture is required to reduce the emission of hydrocarbons (HC) and carbon monoxide (CO) and prevent deceleration backfire. To adjust injector on-time, the PCM uses the decrease in MAP and the decrease in throttle position to calculate a decrease in pulse width.

Deceleration may be either partial or full. That is, the driver may have simply backed off the throttle, or may have allowed the throttle valve to return all the way to the idle position.

It is important to make this distinction because the PCM reacts differently to each of these conditions.

Under light-throttle deceleration, the PCM reduces fuel flow by shortening injector on-time (deceleration enleanment).

Under closed throttle deceleration, the PCM senses that the driver intends the engine to return to idle speed. Fuel delivery may be cut off entirely (deceleration fuel cut-off). As desired idle speed is approached, fuel delivery and idle air control come into play to maintain the desired idle.

So the PCM has to have these non load cells and they would have a leaner trim values then when using a load based fuel trim cell.

Would this be for cells 0, 1, 2, & 21?

Is there an implication in tuning? If so, instead of shooting for a LTFT of -2, would a positive number be a better target?

Yep any cell used when TPS is being backed off the engine then does not need as much fuel so those cells would be biased lean rather then LOAD cells.

The point is people see those non load cells leaner and are tuning them richer when it is non load conditions so do not expect those cells to be at zero LTFTs, so yes there is no reason to tune those cells richer when your backing off the g pedal.