Burnout occurs when a portal has been hacked by an Agent four times within four hours. Burnout lasts for four hours after the first hack, preventing the Agent from hacking the portal within that time. When a player attempts to hack a burnt out portal, an alert will pop up, reading "Portal burnt out! It may take a significant time for portal to reset."
Burnout can be delayed using Multi-hacks, which increase the number of hacks an Agent can make before the portal burns out. Installing a Heat Sink will reset the burnout counter for the Agent who installed it.
Portal burnout is exclusive to one Agent; that is, when one Agent burns out a portal, the portal shows up as burnt out only for them. Other Agents can still hack a portal that was burnt out by another Agent.