diff --git a/bme280.py b/bme280.py
index 1a29a1c..642468a 100644
--- a/bme280.py
+++ b/bme280.py
@@ -33,6 +33,15 @@ def read_id(addr: int = DEVICE_ADDRESS) -> tuple[int, int]:
     return chip_id, chip_version
 
 
+def _wait_nvm_copy(bus: smbus2.SMBus, addr: int) -> None:
+    """Poll status register until NVM copy is complete after soft reset."""
+    for _ in range(5):
+        if not (bus.read_byte_data(addr, 0xF3) & 0x01):
+            return
+        time.sleep(0.002)
+    raise OSError("BME280 NVM copy did not complete after soft reset")
+
+
 def read_all(addr: int = DEVICE_ADDRESS) -> tuple[float, float, float]:
     OVERSAMPLE_TEMP = 2
     OVERSAMPLE_PRES = 2
@@ -40,6 +49,11 @@ def read_all(addr: int = DEVICE_ADDRESS) -> tuple[float, float, float]:
     MODE = 1
 
     with smbus2.SMBus(I2C_BUS) as bus:
+        # Soft reset to ensure the sensor starts from a known state
+        bus.write_byte_data(addr, 0xE0, 0xB6)
+        time.sleep(0.002)  # 2ms startup delay (datasheet section 4.2)
+        _wait_nvm_copy(bus, addr)
+
         bus.write_byte_data(addr, 0xF2, OVERSAMPLE_HUM)
         bus.write_byte_data(addr, 0xF4, OVERSAMPLE_TEMP << 5 | OVERSAMPLE_PRES << 2 | MODE)
 
diff --git a/tests/test_bme280.py b/tests/test_bme280.py
index 0224e1d..6ec2bc9 100644
--- a/tests/test_bme280.py
+++ b/tests/test_bme280.py
@@ -5,6 +5,7 @@
 
 def make_mock_bus(chip_id: int = 96, chip_version: int = 0) -> MagicMock:
     bus = MagicMock()
+    bus.read_byte_data.return_value = 0  # 0xF3 status: NVM copy done, not measuring
     bus.read_i2c_block_data.side_effect = [
         [chip_id, chip_version],   # read_id
         [0] * 24,                  # cal1 - T and P calibration
@@ -107,3 +108,15 @@ def test_pressure_zero_when_p1_calibration_is_zero(patched_smbus):
     import bme280
     result = bme280.sensor()
     assert result['data']['pressure'] == 0.0
+
+
+def test_nvm_copy_timeout_raises_oserror():
+    bus = MagicMock()
+    bus.read_byte_data.return_value = 0x01  # NVM copy never completes
+    with patch('bme280.smbus2.SMBus') as MockSMBus:
+        instance = MockSMBus.return_value
+        instance.__enter__ = MagicMock(return_value=bus)
+        instance.__exit__ = MagicMock(return_value=False)
+        import bme280
+        with pytest.raises(OSError, match="NVM copy"):
+            bme280.read_all()