diff --git a/Core/Inc/AMS_CAN.h b/Core/Inc/AMS_CAN.h
index 7f5911b..31c42a4 100644
--- a/Core/Inc/AMS_CAN.h
+++ b/Core/Inc/AMS_CAN.h
@@ -8,9 +8,15 @@
 #ifndef INC_AMS_CAN_H_
 #define INC_AMS_CAN_H_
 
-#include "stm32f4xx_hal.h"
+#include "main.h"
 
-#define CAN_ID_AMS_SLAVE_HEARTBEAT_BASE 0x100
+#include "stm32f4xx_hal.h"
+#include "stm32f4xx_hal_can.h"
+#include "stm32f4xx_hal_def.h"
+
+#define CAN_ID_SLAVE_ERROR 0x001
+#define CAN_ID_AMS_SLAVE_HEARTBEAT_BASE 0x600
+#define CAN_HEARTBEAT_TX_TIMEOUT 5 /* ms */
 
 void ams_can_init(CAN_HandleTypeDef* ams, CAN_HandleTypeDef* car);
 
@@ -18,5 +24,9 @@ void ams_can_handle_ams_msg(CAN_RxHeaderTypeDef* header, uint8_t* data);
 void ams_can_handle_car_msg(CAN_RxHeaderTypeDef* header, uint8_t* data);
 
 void ams_can_send_heartbeat();
+void ams_can_send_error(AMS_ErrorCode error_code);
+
+HAL_StatusTypeDef ams_can_wait_for_free_mailbox(CAN_HandleTypeDef* handle,
+                                                uint32_t timeout);
 
 #endif /* INC_AMS_CAN_H_ */
diff --git a/Core/Inc/main.h b/Core/Inc/main.h
index 4d993bd..0760271 100644
--- a/Core/Inc/main.h
+++ b/Core/Inc/main.h
@@ -36,11 +36,17 @@ extern "C" {
 
 /* Exported types ------------------------------------------------------------*/
 /* USER CODE BEGIN ET */
-
+typedef enum {
+  AMS_ERROR_UV,
+  AMS_ERROR_OV,
+  AMS_ERROR_UT,
+  AMS_ERROR_OT
+} AMS_ErrorCode;
 /* USER CODE END ET */
 
 /* Exported constants --------------------------------------------------------*/
 /* USER CODE BEGIN EC */
+extern uint8_t slave_id;
 /* USER CODE END EC */
 
 /* Exported macro ------------------------------------------------------------*/
diff --git a/Core/Src/AMS_CAN.c b/Core/Src/AMS_CAN.c
index 0c0bec2..28f9a77 100644
--- a/Core/Src/AMS_CAN.c
+++ b/Core/Src/AMS_CAN.c
@@ -9,6 +9,12 @@
 
 #include "BQ_Abstraction_Layer.h"
 #include "TMP144.h"
+#include "common_defs.h"
+#include "main.h"
+
+#include "stm32f4xx.h"
+#include "stm32f4xx_hal.h"
+#include "stm32f4xx_hal_can.h"
 
 static CAN_HandleTypeDef* handle_ams;
 static CAN_HandleTypeDef* handle_car;
@@ -81,34 +87,71 @@ void ams_can_handle_car_msg(CAN_RxHeaderTypeDef* header, uint8_t* data) {}
 void ams_can_send_heartbeat() {
   static CAN_TxHeaderTypeDef header;
   static uint8_t data[8];
-  static uint8_t SLAVE_ID = 0;
 
   header.IDE = CAN_ID_STD;
   header.DLC = 8;
   header.RTR = CAN_RTR_DATA;
   header.TransmitGlobalTime = DISABLE;
 
-  for (int i = 0; i < N_CELLS / 2; i++) {
-    header.StdId = CAN_ID_AMS_SLAVE_HEARTBEAT_BASE | (SLAVE_ID) << 4 | i;
-    uint16_t v1 = cell_voltages[i * 2];
-    uint16_t v2 = cell_voltages[i * 2 + 1];
-    data[0] = v1 >> 8;
-    data[1] = v1 & 0xFF;
-    data[2] = v2 >> 8;
-    data[3] = v2 & 0xFF;
-    uint16_t t1 = temperatures[i * 2];
-    uint16_t t2 = temperatures[i * 2 + 1];
-    data[4] = t1 >> 8;
-    data[5] = t1 & 0xFF;
-    data[6] = t2 >> 8;
-    data[7] = t2 & 0xFF;
-    uint32_t mailbox;
-    HAL_StatusTypeDef status;
-    if ((status = HAL_CAN_AddTxMessage(handle_ams, &header, data, &mailbox)) !=
+  // Send voltages
+  for (int msg_id = 0; msg_id < 3; msg_id++) {
+    header.StdId = CAN_ID_AMS_SLAVE_HEARTBEAT_BASE | (slave_id << 4) | msg_id;
+    for (int i = 0; i < 4; i++) {
+      int cell = msg_id * 4 + i;
+      uint16_t v = (cell < N_CELLS) ? cell_voltages[cell] : 0;
+      data[2 * i + 0] = v & 0xFF;
+      data[2 * i + 1] = v >> 8;
+    }
+    if (ams_can_wait_for_free_mailbox(handle_ams, CAN_HEARTBEAT_TX_TIMEOUT) ==
         HAL_OK) {
-      Error_Handler();
-    } else {
-      HAL_Delay(100);
+      uint32_t mailbox;
+      HAL_CAN_AddTxMessage(handle_ams, &header, data, &mailbox);
+    }
+  }
+
+  // Send temperatures
+  for (int temp_msg_id = 0; temp_msg_id < 8; temp_msg_id++) {
+    int msg_id = temp_msg_id + 3;
+    header.StdId = CAN_ID_AMS_SLAVE_HEARTBEAT_BASE | (slave_id << 4) | msg_id;
+    for (int i = 0; i < 4; i++) {
+      int sensor = temp_msg_id * 4 + i;
+      uint16_t temp = temperatures[sensor];
+      data[2 * i + 0] = temp & 0xFF;
+      data[2 * i + 1] = temp >> 8;
+    }
+    if (ams_can_wait_for_free_mailbox(handle_ams, CAN_HEARTBEAT_TX_TIMEOUT) ==
+        HAL_OK) {
+      uint32_t mailbox;
+      HAL_CAN_AddTxMessage(handle_ams, &header, data, &mailbox);
     }
   }
 }
+
+void ams_can_send_error(AMS_ErrorCode error_code) {
+  static CAN_TxHeaderTypeDef header;
+  header.IDE = CAN_ID_STD;
+  header.DLC = 8;
+  header.RTR = CAN_RTR_DATA;
+  header.TransmitGlobalTime = DISABLE;
+  header.StdId = CAN_ID_SLAVE_ERROR;
+
+  static uint8_t data[8];
+  data[0] = slave_id;
+  data[1] = error_code;
+
+  HAL_CAN_AbortTxRequest(handle_ams,
+                         CAN_TX_MAILBOX0 | CAN_TX_MAILBOX1 | CAN_TX_MAILBOX2);
+  uint32_t mailbox;
+  HAL_CAN_AddTxMessage(handle_ams, &header, data, &mailbox);
+}
+
+HAL_StatusTypeDef ams_can_wait_for_free_mailbox(CAN_HandleTypeDef* handle,
+                                                uint32_t timeout) {
+  uint32_t end = HAL_GetTick() + timeout;
+  while (HAL_GetTick() < end) {
+    if (HAL_CAN_GetTxMailboxesFreeLevel(handle) > 0) {
+      return HAL_OK;
+    }
+  }
+  return HAL_TIMEOUT;
+}
diff --git a/Core/Src/main.c b/Core/Src/main.c
index 16cdcbe..3c8d85d 100644
--- a/Core/Src/main.c
+++ b/Core/Src/main.c
@@ -60,7 +60,7 @@ UART_HandleTypeDef huart3;
 UART_HandleTypeDef huart6;
 
 /* USER CODE BEGIN PV */
-
+uint8_t slave_id;
 /* USER CODE END PV */
 
 /* Private function prototypes -----------------------------------------------*/
@@ -149,10 +149,7 @@ int main(void) {
   HAL_Delay(100);
   afe_init(&huart2);
   eeprom_init(&hi2c1);
-  uint8_t id;
-  if (eeprom_read_random(EEPROM_ADDR_SLAVE_ID, &id) == HAL_OK) {
-    HAL_GPIO_WritePin(STAT_LED4_GPIO_Port, STAT_LED4_Pin, GPIO_PIN_SET);
-  }
+  eeprom_read_random(EEPROM_ADDR_SLAVE_ID, &slave_id);
   fan_ctrl_init(&htim3, TIM_CHANNEL_4);
   /* USER CODE END 2 */