There are two types of events that will make a PRTG sensor change state, between okay, warning and error. The first is when a device stops responding at all, for example, if you’re collecting traffic statistics using SNMP, and the device stops responding to SNMP. The first time PRTG doesn’t receive a reply, the sensor will go into warning state. The second time PRTG doesn’t receive a reply, the sensor will go into error state. If you’d like to adjust this behavior, to go directly to error state, for example, you can change this under the Scanning Interval settings, at any point in your device tree hierarchy. Settings / Scanning Interval / When a sensor reports an error The other event that will set a sensor into warning or error state is when the measured values cross one of the thresholds you have set. These thresholds are called Limits in PRTG. You can set custom limits for every sensor and every channel in each sensor within PRTG. You can set upper and lower warning limits and error limits. When the values cross your warning limit, the sensor will go into warning state immediately. And when the values cross your error limit, the sensor will go into error state immediately. To set warning and error limits, go to the sensor, then click on the gear icon for the channel. Click on “Enable Limits” and then fill in the upper and lower limits you would like to have.