diff --git a/co2_sensor.cpp b/co2_sensor.cpp
index af4807e478918a597222cd06149cd7b67f35c46f..5eb0f7b34a03780452c8a8c95308eca985c7ef81 100644
--- a/co2_sensor.cpp
+++ b/co2_sensor.cpp
@@ -84,7 +84,7 @@ namespace sensor {
     static int16_t previous_co2 = 0;
     if (co2 > (previous_co2 - 30) && co2 < (previous_co2 + 30)) {
       stable_measurements++;
-      Serial.print("Stable since : ");
+      Serial.print(F("Number of stable measurements : "));
       Serial.println(stable_measurements);
       waiting_color = color::green;
     } else {
diff --git a/led_effects.cpp b/led_effects.cpp
index 6eb91d3521a0c17c84f6e1a5f0179aa36993c49a..3804ee1111cd21cd4e4c5a73b900dec772d046e6 100644
--- a/led_effects.cpp
+++ b/led_effects.cpp
@@ -182,7 +182,7 @@ namespace LedEffects {
    */
   int countdownToZero() {
     if (config::night_mode) {
-      Serial.println("Night mode. Not doing anything.");
+      Serial.println(F("Night mode. Not doing anything."));
       delay(1000); // Wait for a while, to avoid coming back to this function too many times when button is pressed.
       return 1;
     }