diff --git a/libraries/AP_HAL_PX4/I2CDevice.cpp b/libraries/AP_HAL_PX4/I2CDevice.cpp
index 5b28ab9bd9..624aa7439b 100644
--- a/libraries/AP_HAL_PX4/I2CDevice.cpp
+++ b/libraries/AP_HAL_PX4/I2CDevice.cpp
@@ -53,7 +53,13 @@ uint8_t PX4_I2C::map_bus_number(uint8_t bus) const
 #else
         return 1;
 #endif
-        
+    case 2:
+        // map to expansion bus 2
+#ifdef PX4_I2C_BUS_EXPANSION1
+        return PX4_I2C_BUS_EXPANSION1;
+#else
+        return 2;
+#endif
     }
     // default to bus 1
     return 1;
diff --git a/libraries/AP_HAL_PX4/I2CDevice.h b/libraries/AP_HAL_PX4/I2CDevice.h
index 0246dd1ea9..12f0a5eb80 100644
--- a/libraries/AP_HAL_PX4/I2CDevice.h
+++ b/libraries/AP_HAL_PX4/I2CDevice.h
@@ -70,7 +70,7 @@ public:
     }
     
 private:
-    static const uint8_t num_buses = 2;
+    static const uint8_t num_buses = 3;
     static DeviceBus businfo[num_buses];
     
     uint8_t _busnum;