# Is a BLTouch probe used during a print after the first layer?

I was looking at a BLTouch probe, and saw the recent question about one with intermittent failure. That was kind of scary, where I might spend a bunch of money for something that would make my printer harder to use, rather than easier.

But I was curious. I don't have any personal experience using these, but it seems like the probe would only be used during an initial level and when homing the Z axis. Thus, as long as the probe works most of the time, and failure to home properly doesn't damage your nozzle or print bed, maybe this kind of thing isn't too bad.

Is this assessment of how the probe functions (that it's not really used after the start of a print) accurate?

In the spirit of stack exchange, I'm not interested in the subjective issue of nozzle or print bed damage; I'm only asking about the objective assessment of how a probe is used by the printer.

The probe is used by G-code command G29; this command is used by a few printer firmwares to perform a bed probing sequence where, depending on the options set in the firmware, the surface of the bed is scanned by deploying the sensor at various locations. From these measurements, the firmware will determine the shape or tilt of the bed to compensate for deviations in Z height during the first couple of layers or millimeters of the print. Basically, it will keep your nozzle (approximately, as it is based on calculations) at the same distance all over the bed area.
Once the print starts after the G29 command, the sensor is stowed until the next print calls the G29 command (or M280 with arguments to deploy on Marlin/Duet/Smoothieware, or M340 on Repetier firmware).