diff --git a/fw/Core/Src/main.c b/fw/Core/Src/main.c
index a291fcf..a058d38 100644
--- a/fw/Core/Src/main.c
+++ b/fw/Core/Src/main.c
@@ -331,9 +331,10 @@ int main(void)
 		  /* Read SCD4x data (if connected) */
 		  if (scd4x_is_connected == 1)
 		  {
-			  scd4x_read_measurement(&CO2,
-					  	  	  	  	 &T_SCD4x,
-									 &RH_SCD4x);
+			  if (scd4x_read_measurement(&CO2, &T_SCD4x, &RH_SCD4x) != SCD4X_OK) {
+				  /* read failed, either I2C fail or CRC error */
+				  // TODO something
+			  }
 			  if (CO2 > 0) {
 				  co2_valid = 1;
 			  } else {
diff --git a/fw/Core/Src/scd4x.c b/fw/Core/Src/scd4x.c
index e5c4645..6452c3a 100644
--- a/fw/Core/Src/scd4x.c
+++ b/fw/Core/Src/scd4x.c
@@ -80,11 +80,9 @@ int8_t scd4x_read_measurement(uint16_t *co2, int16_t *temperature, uint16_t *rel
 	buffer[0] = SCD4X_READ_MEASUREMENT >> 8;
 	buffer[1] = SCD4X_READ_MEASUREMENT & 0x00ff;
 	result = i2c_transmit(SCD4X_I2C_ADDRESS<<1, buffer, 2);
-
-	// TODO: Proc to vraci NACK? Vyresit.
-	/*if (result != I2C_OK) {
+	if (result != I2C_OK) {
 		return SCD4X_ERROR;
-	}*/
+	}
 	uart_enable_interrupts();
 	LL_mDelay(1); // 10 ms should be enough
 	uart_disable_interrupts();
@@ -95,12 +93,22 @@ int8_t scd4x_read_measurement(uint16_t *co2, int16_t *temperature, uint16_t *rel
 	{
 		return SCD4X_ERROR;
 	}
-
-	// TODO checksum
-	// Convert to T and RH; taken directly from pseudocode in SHT4x datasheet, page 3
+	/* Convert to T and RH; taken directly from pseudocode in SHT4x datasheet, page 3 */
 	uint32_t co2_ticks = (buffer[0] << 8) + buffer[1];
+	uint8_t co2_crc = buffer[2];
 	uint32_t t_ticks = (buffer[3] << 8) + buffer[4];
+	uint8_t t_crc = buffer[5];
 	uint32_t rh_ticks = (buffer[6] << 8) + buffer[7];
+	uint8_t rh_crc = buffer[8];
+
+	/* check CRC-8 checksum */
+	uint8_t crc_correct = crc8_calculate(buffer, 2) == co2_crc;
+	crc_correct &= crc8_calculate(buffer + 3, 2) == t_crc;
+	crc_correct &= crc8_calculate(buffer + 6, 2) == rh_crc;
+	if (!crc_correct) {
+		return SCD4X_CRC8_ERROR;
+	}
+	/* copy to output variables */
 	int t_degC = -450 + 10 * 175 * t_ticks / 65535;
 	int rh_pRH = 100 * rh_ticks / 65535;
 	if (rh_pRH > 100) {