diff --git a/enviro/__init__.py b/enviro/__init__.py
index 7f0ca48..4a05659 100644
--- a/enviro/__init__.py
+++ b/enviro/__init__.py
@@ -10,9 +10,9 @@
 i2c = PimoroniI2C(I2C_SDA_PIN, I2C_SCL_PIN, 100000)
 i2c_devices = i2c.scan()
 model = None
-if 56 in i2c_devices: # 56 = colour / light sensor and only present on Indoor
+if I2C_ADDR_BH1745 in i2c_devices: # colour / light sensor and only present on Indoor
   model = "indoor"
-elif 35 in i2c_devices: # 35 = ltr-599 on grow & weather
+elif I2C_ADDR_LTR559 in i2c_devices: # ltr-599 on grow & weather
   pump3_pin = Pin(12, Pin.IN, Pin.PULL_UP)
   model = "grow" if pump3_pin.value() == False else "weather"    
   pump3_pin.init(pull=None)
@@ -33,9 +33,13 @@ def get_board():
 
 # return any additional sensors connected with Qw/ST
 def get_additional_sensors():
-  if 98 in i2c_devices:
-    import enviro.sensors.scd41 as scd41
-    yield scd41
+  if I2C_ADDR_SCD41 in i2c_devices:
+    try:
+      import enviro.sensors.scd41 as scd41
+      yield scd41
+    except RuntimeError:
+      # Likely another device present on the SCD41 address
+      pass
 
 # set up the activity led
 # ===========================================================================
diff --git a/enviro/constants.py b/enviro/constants.py
index 0dadcea..f57f153 100644
--- a/enviro/constants.py
+++ b/enviro/constants.py
@@ -40,3 +40,8 @@
 UPLOAD_SUCCESS = 0
 UPLOAD_FAILED = 1
 UPLOAD_RATE_LIMITED = 2
+
+# I2C addresses
+I2C_ADDR_BH1745   = 0x38
+I2C_ADDR_LTR559   = 0x23
+I2C_ADDR_SCD41    = 0x62