diff --git a/board/drivers/can.h b/board/drivers/can.h
index d1b9de1be4b5e1..dbc1eb85d9d3d3 100644
--- a/board/drivers/can.h
+++ b/board/drivers/can.h
@@ -156,48 +156,45 @@ void can_init_all(void) {
   }
 }
 
-void can_set_gmlan(int bus) {
-  if ((bus == -1) || (bus != can_num_lookup[3])) {
-    // GMLAN OFF
-    switch (can_num_lookup[3]) {
+void can_set_gmlan(uint8_t bus) {
+
+  // first, disable GMLAN on prev bus
+  uint8_t prev_bus = can_num_lookup[3];
+  if (bus != prev_bus) {
+    switch (prev_bus) {
       case 1:
-        puts("disable GMLAN on CAN2\n");
-        set_can_mode(1, 0);
-        bus_lookup[1] = 1;
-        can_num_lookup[1] = 1;
-        can_num_lookup[3] = -1;
-        can_init(1);
-        break;
       case 2:
-        puts("disable GMLAN on CAN3\n");
-        set_can_mode(2, 0);
-        bus_lookup[2] = 2;
-        can_num_lookup[2] = 2;
+        puts("Disable GMLAN on CAN");
+        puth(prev_bus + 1U);
+        puts("\n");
+        set_can_mode(prev_bus, 0);
+        bus_lookup[prev_bus] = prev_bus;
+        can_num_lookup[prev_bus] = prev_bus;
         can_num_lookup[3] = -1;
-        can_init(2);
+        can_init(prev_bus);
         break;
       default:
-        puts("GMLAN bus value invalid\n");
+        // GMLAN was not set on either BUS 1 or 2
         break;
     }
   }
 
-  if (bus == 1) {
-    puts("GMLAN on CAN2\n");
-    // GMLAN on CAN2
-    set_can_mode(1, 1);
-    bus_lookup[1] = 3;
-    can_num_lookup[1] = -1;
-    can_num_lookup[3] = 1;
-    can_init(1);
-  } else if (bus == 2) {
-    puts("GMLAN on CAN3\n");
-    // GMLAN on CAN3
-    set_can_mode(2, 1);
-    bus_lookup[2] = 3;
-    can_num_lookup[2] = -1;
-    can_num_lookup[3] = 2;
-    can_init(2);
+  // now enable GMLAN on the new bus
+  switch (bus) {
+    case 1:
+    case 2:
+      puts("Enable GMLAN on CAN");
+      puth(bus + 1U);
+      puts("\n");
+      set_can_mode(bus, 1);
+      bus_lookup[bus] = 3;
+      can_num_lookup[bus] = -1;
+      can_num_lookup[3] = bus;
+      can_init(bus);
+      break;
+    default:
+      puts("GMLAN can only be set on CAN2 or CAN3");
+      break;
   }
 }
 
diff --git a/board/drivers/gmlan_alt.h b/board/drivers/gmlan_alt.h
index c3d5d304e63c48..b362a95bf7eb7a 100644
--- a/board/drivers/gmlan_alt.h
+++ b/board/drivers/gmlan_alt.h
@@ -196,7 +196,7 @@ void TIM4_IRQHandler(void) {
         } else {
           gmlan_silent_count++;
         }
-      } else if (gmlan_silent_count == REQUIRED_SILENT_TIME) {
+      } else {
         bool retry = 0;
         // in send loop
         if ((gmlan_sending > 0) &&  // not first bit
@@ -209,6 +209,8 @@ void TIM4_IRQHandler(void) {
         } else if ((read == 1) && (gmlan_sending == (gmlan_sendmax - 11))) {    // recessive during ACK
           puts("GMLAN ERR: didn't recv ACK\n");
           retry = 1;
+        } else {
+          // do not retry
         }
         if (retry) {
           // reset sender (retry after 7 silent)
@@ -233,9 +235,7 @@ void TIM4_IRQHandler(void) {
       }
     }
     TIM4->SR = 0;
-  } //bit bang mode
-
-  else if (gmlan_alt_mode == GPIO_SWITCH) {
+  } else if (gmlan_alt_mode == GPIO_SWITCH) {
     if ((TIM4->SR & TIM_SR_UIF) && (gmlan_switch_below_timeout != -1)) {
       if ((can_timeout_counter == 0) && gmlan_switch_timeout_enable) {
         //it has been more than 1 second since timeout was reset; disable timer and restore the GMLAN output
@@ -258,7 +258,9 @@ void TIM4_IRQHandler(void) {
       }
     }
     TIM4->SR = 0;
-  } //gmlan switch mode
+  } else {
+    puts("invalid gmlan_alt_mode\n");
+  }
 }
 
 void bitbang_gmlan(CAN_FIFOMailBox_TypeDef *to_bang) {
diff --git a/board/drivers/llcan.h b/board/drivers/llcan.h
index 5d0b8884583afa..293f40dfe41780 100644
--- a/board/drivers/llcan.h
+++ b/board/drivers/llcan.h
@@ -8,6 +8,8 @@
 // 5000 = 500 kbps
 #define can_speed_to_prescaler(x) (CAN_PCLK / CAN_QUANTA * 10 / (x))
 
+void puts(const char *a);
+
 bool llcan_set_speed(CAN_TypeDef *CAN, uint32_t speed, bool loopback, bool silent) {
   // initialization mode
   CAN->MCR = CAN_MCR_TTCM | CAN_MCR_INRQ;
@@ -70,6 +72,8 @@ void llcan_init(CAN_TypeDef *CAN) {
     NVIC_EnableIRQ(CAN3_RX0_IRQn);
     NVIC_EnableIRQ(CAN3_SCE_IRQn);
 #endif
+  } else {
+    puts("Invalid CAN: initialization failed\n");
   }
 }
 
diff --git a/board/drivers/uart.h b/board/drivers/uart.h
index 56453715aa701c..6eef563e1e1c88 100644
--- a/board/drivers/uart.h
+++ b/board/drivers/uart.h
@@ -289,6 +289,8 @@ void uart_init(USART_TypeDef *u, int baud) {
     NVIC_EnableIRQ(USART3_IRQn);
   } else if (u == UART5) {
     NVIC_EnableIRQ(UART5_IRQn);
+  } else {
+    // USART type undefined, skip
   }
 }
 
diff --git a/board/drivers/usb.h b/board/drivers/usb.h
index aadd52811a10dc..ae82b276543d83 100644
--- a/board/drivers/usb.h
+++ b/board/drivers/usb.h
@@ -793,18 +793,19 @@ void usb_irqhandler(void) {
   if ((gintsts & USB_OTG_GINTSTS_RXFLVL) != 0) {
     // 1. Read the Receive status pop register
     volatile unsigned int rxst = USBx->GRXSTSP;
+    int status = (rxst & USB_OTG_GRXSTSP_PKTSTS) >> 17;
 
     #ifdef DEBUG_USB
       puts(" RX FIFO:");
       puth(rxst);
       puts(" status: ");
-      puth((rxst & USB_OTG_GRXSTSP_PKTSTS) >> 17);
+      puth(status);
       puts(" len: ");
       puth((rxst & USB_OTG_GRXSTSP_BCNT) >> 4);
       puts("\n");
     #endif
 
-    if (((rxst & USB_OTG_GRXSTSP_PKTSTS) >> 17) == STS_DATA_UPDT) {
+    if (status == STS_DATA_UPDT) {
       int endpoint = (rxst & USB_OTG_GRXSTSP_EPNUM);
       int len = (rxst & USB_OTG_GRXSTSP_BCNT) >> 4;
       (void)USB_ReadPacket(&usbdata, len);
@@ -822,13 +823,15 @@ void usb_irqhandler(void) {
       if (endpoint == 3) {
         usb_cb_ep3_out(usbdata, len, 1);
       }
-    } else if (((rxst & USB_OTG_GRXSTSP_PKTSTS) >> 17) == STS_SETUP_UPDT) {
+    } else if (status == STS_SETUP_UPDT) {
       (void)USB_ReadPacket(&setup, 8);
       #ifdef DEBUG_USB
         puts("  setup ");
         hexdump(&setup, 8);
         puts("\n");
       #endif
+    } else {
+      // status is neither STS_DATA_UPDT or STS_SETUP_UPDT, skip
     }
   }
 
@@ -903,6 +906,8 @@ void usb_irqhandler(void) {
       puts("OUTEP3 error ");
       puth(USBx_OUTEP(3)->DOEPINT);
       puts("\n");
+    } else {
+      // USBx_OUTEP(3)->DOEPINT is 0, ok to skip
     }
 
     if ((USBx_OUTEP(0)->DOEPINT & USB_OTG_DIEPINT_XFRC) != 0) {
diff --git a/board/gpio.h b/board/gpio.h
index 26a82d6275e05f..255789a0414b30 100644
--- a/board/gpio.h
+++ b/board/gpio.h
@@ -127,6 +127,8 @@ void set_can_enable(CAN_TypeDef *CAN, bool enabled) {
     // CAN3_EN
     set_gpio_output(GPIOA, 0, !enabled);
   #endif
+  } else {
+    puts("Invalid CAN: enabling failed\n");
   }
 }
 
@@ -171,6 +173,8 @@ void set_can_mode(int can, bool use_gmlan) {
       set_gpio_alternate(GPIOB, 3, GPIO_AF11_CAN3);
       set_gpio_alternate(GPIOB, 4, GPIO_AF11_CAN3);
 #endif
+    } else {
+      puts("Invalid CAN: mode setting failed\n");
     }
   } else {
     if (can == 1) {
@@ -190,6 +194,8 @@ void set_can_mode(int can, bool use_gmlan) {
       set_gpio_alternate(GPIOA, 8, GPIO_AF11_CAN3);
       set_gpio_alternate(GPIOA, 15, GPIO_AF11_CAN3);
 #endif
+    } else {
+      puts("Invalid CAN: mode setting failed\n");
     }
   }
 }
diff --git a/board/main.c b/board/main.c
index 3e3820e440a874..810b09286b7867 100644
--- a/board/main.c
+++ b/board/main.c
@@ -285,6 +285,8 @@ int usb_cb_control_msg(USB_Setup_TypeDef *setup, uint8_t *resp, bool hardwired)
           can_set_gmlan(1);
         } else if (setup->b.wIndex.w == 2) {
           can_set_gmlan(2);
+        } else {
+          puts("Invalid bus num for GMLAN CAN set\n");
         }
       } else {
         can_set_gmlan(-1);
@@ -332,6 +334,8 @@ int usb_cb_control_msg(USB_Setup_TypeDef *setup, uint8_t *resp, bool hardwired)
         can_set_forwarding(setup->b.wValue.w, setup->b.wIndex.w & CAN_BUS_NUM_MASK);
       } else if((setup->b.wValue.w < BUS_MAX) && (setup->b.wIndex.w == 0xFF)){ //Clear Forwarding
         can_set_forwarding(setup->b.wValue.w, -1);
+      } else {
+        puts("Invalid CAN bus forwarding\n");
       }
       break;
     // **** 0xde: set can bitrate
@@ -462,6 +466,8 @@ int usb_cb_control_msg(USB_Setup_TypeDef *setup, uint8_t *resp, bool hardwired)
       } else if (setup->b.wValue.w < BUS_MAX) {
         puts("Clearing CAN Tx queue\n");
         can_clear(can_queues[setup->b.wValue.w]);
+      } else {
+        puts("Clearing CAN CAN ring buffer failed: wrong bus number\n");
       }
       break;
     // **** 0xf2: Clear UART ring buffer.