diff --git a/ampel-firmware/co2_sensor.cpp b/ampel-firmware/co2_sensor.cpp
index 8a031b1508130baa15b72f20a0ad93fe17c9ad7e..a41162aff8c255ff501c6ba7d0bc018d0b885a48 100644
--- a/ampel-firmware/co2_sensor.cpp
+++ b/ampel-firmware/co2_sensor.cpp
@@ -216,6 +216,7 @@ namespace sensor {
       if (co2 <= 0) {
         // NOTE: Data is available, but it's sometimes erroneous: the sensor outputs
         // zero ppm but non-zero temperature and non-zero humidity.
+        Serial.println(F("Invalid sensor data - CO2 concentration <= 0 ppm"));
         switchState(BOOTUP);
       } else if (co2 < 250) {
         // Sensor should be calibrated.