diff --git a/MHZ.cpp b/MHZ.cpp
index b4af86a..44f005e 100644
--- a/MHZ.cpp
+++ b/MHZ.cpp
@@ -5,6 +5,16 @@
 
 #include "MHZ.h"
 
+const int MHZ14A = 14;
+const int MHZ19B = 19;
+
+const int STATUS_NO_RESPONSE = -2;
+const int STATUS_CHECKSUM_MISMATCH = -3;
+const int STATUS_INCOMPLETE = -4;
+const int STATUS_NOT_READY = -5;
+
+unsigned long lastRequest = 0;
+
 MHZ::MHZ(uint8_t rxpin, uint8_t txpin, uint8_t pwmpin, uint8_t type)
     : co2Serial(rxpin, txpin) {
   _rxpin = rxpin;
@@ -15,6 +25,9 @@ MHZ::MHZ(uint8_t rxpin, uint8_t txpin, uint8_t pwmpin, uint8_t type)
   co2Serial.begin(9600);
 }
 
+/**
+ * Enables or disables the debug mode (more logging).
+ */
 void MHZ::setDebug(boolean enable) {
   debug = enable;
   if (debug) {
@@ -24,15 +37,40 @@ void MHZ::setDebug(boolean enable) {
   }
 }
 
-int retryCount = 0;
+boolean MHZ::isPreHeating() {
+  if (_type == MHZ14A) {
+    return millis() < (3 * 60 * 1000);
+  } else if (_type == MHZ19B) {
+    return millis() < (3 * 60 * 1000);
+  } else {
+    Serial.println("MHZ::isPreHeating() => UNKNOWN SENSOR");
+    return false;
+  }
+}
+
+boolean MHZ::isReady() {
+  if (isPreHeating()) return false;
+  if (_type == MHZ14A)
+    return lastRequest < millis() - 90 * 1000;
+  else if (_type == MHZ19B)
+    return lastRequest < millis() - 60 * 1000;
+  else {
+    Serial.print("MHZ::isReady() => UNKNOWN SENSOR \"");
+    Serial.print(_type);
+    Serial.println("\"");
+    return true;
+  }
+}
 
 int MHZ::readCO2UART() {
+  if (!isReady()) return STATUS_NOT_READY;
   if (debug) Serial.println("-- read CO2 uart ---");
   byte cmd[9] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79};
   byte response[9];  // for answer
 
   if (debug) Serial.print("  >> Sending CO2 request");
   co2Serial.write(cmd, 9);  // request PPM CO2
+  lastRequest = millis();
 
   // clear the buffer
   memset(response, 0, 9);
@@ -124,9 +162,12 @@ int MHZ::readCO2UART() {
   return ppm_uart;
 }
 
-uint8_t MHZ::getLastTemperature() { return temperature; }
+uint8_t MHZ::getLastTemperature() {
+  if (!isReady()) return STATUS_NOT_READY;
+  return temperature;
+}
 
-byte MHZ::getCheckSum(byte *packet) {
+byte MHZ::getCheckSum(byte* packet) {
   if (debug) Serial.println("  getCheckSum()");
   byte i;
   unsigned char checksum = 0;
@@ -139,6 +180,7 @@ byte MHZ::getCheckSum(byte *packet) {
 }
 
 int MHZ::readCO2PWM() {
+  // if (!isReady()) return STATUS_NOT_READY; not needed?
   if (debug) Serial.print("-- reading CO2 from pwm ");
   unsigned long th, tl, ppm_pwm = 0;
   do {
diff --git a/MHZ.h b/MHZ.h
index b022aca..2af5e29 100644
--- a/MHZ.h
+++ b/MHZ.h
@@ -11,16 +11,18 @@
 #include "WProgram.h"
 #endif
 
-// Define types of sensors.
-#define MHZ14A 14
-#define MHZ19B 19
-
-#define STATUS_NO_RESPONSE -2
-#define STATUS_CHECKSUM_MISMATCH -3
-#define STATUS_INCOMPLETE -4
-
 #include <SoftwareSerial.h>
 
+// types of sensors.
+extern const int MHZ14A;
+extern const int MHZ19B;
+
+// status codes
+extern const int STATUS_NO_RESPONSE;
+extern const int STATUS_CHECKSUM_MISMATCH;
+extern const int STATUS_INCOMPLETE;
+extern const int STATUS_NOT_READY;
+
 class MHZ {
  public:
   MHZ(uint8_t rxpin, uint8_t txpin, uint8_t pwmpin, uint8_t type);
diff --git a/keywords.txt b/keywords.txt
new file mode 100644
index 0000000..35f4450
--- /dev/null
+++ b/keywords.txt
@@ -0,0 +1,41 @@
+#######################################
+# Syntax Coloring Map For MHZ
+#######################################
+
+#######################################
+# Datatypes (KEYWORD1)
+#######################################
+
+MHZ	KEYWORD1
+
+
+#######################################
+# Methods and Functions (KEYWORD2)
+#######################################
+
+setDebug	KEYWORD2
+isPreHeating	KEYWORD2
+isReady	KEYWORD2
+readCO2UART	KEYWORD2
+readCO2PWM	KEYWORD2
+getLastTemperature	KEYWORD2
+
+
+#######################################
+# Instances (KEYWORD2)
+#######################################
+
+co2 KEYWORD2
+
+
+#######################################
+# Constants (LITERAL1)
+#######################################
+
+MHZ14A  LITERAL1
+MHZ19B  LITERAL1
+
+STATUS_NO_RESPONSE  LITERAL1
+STATUS_CHECKSUM_MISMATCH  LITERAL1
+STATUS_INCOMPLETE LITERAL1
+STATUS_NOT_READY  LITERAL1
\ No newline at end of file