diff --git a/src/drivers/hmc5883/hmc5883_i2c.cpp b/src/drivers/hmc5883/hmc5883_i2c.cpp
index d29ffbe0b4..339115c2fd 100644
--- a/src/drivers/hmc5883/hmc5883_i2c.cpp
+++ b/src/drivers/hmc5883/hmc5883_i2c.cpp
@@ -113,17 +113,16 @@ HMC5883_I2C::ioctl(unsigned operation, unsigned &arg)
 	switch (operation) {
 
 	case MAGIOCGEXTERNAL:
-// On PX4v1 the MAG can be on an internal I2C
-// On everything else its always external
-#ifdef CONFIG_ARCH_BOARD_PX4FMU_V1
-		if (_bus == PX4_I2C_BUS_EXPANSION) {
-			return 1;
+#ifdef PX4_I2C_BUS_ONBOARD
+		if (_bus == PX4_I2C_BUS_ONBOARD) {
+			return 0;
 
 		} else {
-			return 0;
+			return 1;
 		}
 
 #else
+		/* assume external for all boards that don't define PX4_I2C_BUS_ONBOARD */
 		return 1;
 #endif