When most digital avalanche transceivers are turned on, they perform a self test. If an error is encountered, most transceivers will either display an error number on the screen, change the color of an indicator light, or blink an indicator light.
The Pieps DSPs (and maybe some other transceivers) uses a tiny fourth antenna to transmit a weak signal during the self test. This allows it to test that the other three antennas can receive a signal. That's a great feature. However, if you turn it on when it's close to a transmitting beacon or an electrical device, the DSP may report an error during the self-test because it is receiving an unexpected signal. People sometimes misinterpret this as a problem with the transceiver when in fact, it is indicative of a thorough self-test. To avoid these phantom errors, you should be at least 5 meters from any electrical devices or transmitting beacons when you turn on your Pieps DSP.
The ARVA Evolution+, Advanced, 3Evo, and 3Axes perform self tests every five minutes to check the battery strength, the transmitting frequency, the transmitting power, etc. If a problem is detected the ARVAs will emit three long beeps every five minutes. That's not very intuitive, but you'll know something is wrong.
Many transceivers, including the Ortovox 3+, Ortovox S1 and the Mammut Element and Pulse have "partner" or "group check" modes that check your partner's transceiver. These mode typically require your partner's transceiver to get within about one meter of the transceiver before receiving a signal. The S1's manual states that during this test is it is checking the frequency, "transmission period," and "period length."