What happens when the CGM stops sending data?

The algorithm needs glucose readings to work. Sometimes it doesn't get them: during sensor warm-up, at the end of a sensor's life, or when the signal is interrupted — for example, distance between components or another communication problem.

The system may continue delivering insulin when full automation is unavailable, but its behavior and on-screen terminology differ by product. Some fall back to preset basal rates, some limit automation, some deliver insulin based on recent history.

Can the reading be trusted?

This is different from the CGM going quiet. Sometimes the CGM is sending a reading, but the reading may not accurately reflect current blood glucose. One example is a compression-related low, where pressure on the sensor produces a falsely low value.

This matters because the system may reduce or suspend insulin in response to a low that isn't real.

Does the system correct highs on its own, or does the person still act?

Some AID systems only adjust background insulin. Others also deliver automatic correction doses when glucose runs high. And some expect the person to initiate a correction through the system's own bolus calculator. These are not the same, and the word “target” doesn't tell you which — you have to know the specific system.

The risk isn't correcting highs; it's giving a correction outside the system's recommended workflow. An additional dose given that way may overlap with insulin already delivered, increasing the risk of insulin stacking — the cumulative effect of overlapping insulin doses — and hypoglycemia.

Who handles meals?

In nearly every current system, the person still gives the system information about meals. But how differs. Some systems ask the person to enter carbohydrate amounts and initiate the system's recommended meal bolus, like a traditional pump. Others have the person announce the meal — “usual,” “more” or “less” — and the system determines the dose. The iLet, for example, independently determines meal doses from a meal-size announcement rather than conventional carbohydrate counting.

Meal timing also differs. Some systems use a bolus before eating; the iLet tells people to announce the meal when they start eating, not before, and not to announce if too much time has passed.

What about exercise?

Physical activity can be challenging because the system responds to glucose data but does not know the person's activity plans — unless the person uses the appropriate feature or adjusts the plan as instructed. Most systems offer an activity mode or a temporary higher target to reduce insulin around exercise.