What does dr error mean on Balboa MVP260 Touchpad and how do I fix this?

Meaning: Possible inadequate water, poor flow, or air bubbles in detected in the heater. Spa is shut down for 15 minutes.

Action Required: Check the water level in the spa. Refill if necessary. If the water level is okay, make sure the pumps have been primed. Press any button to reset, or this message will automatically reset within 15 minutes. If the problem persists, contact your dealer or service organisation.

For all Diagnostic Messages, click on below link.