handle DTC count byte in CAN GET_DTC responses

Author: Brendan Whitfield

obd/protocols/protocol_can.py

@@ -188,8 +188,13 @@ def create_message(self, frames, tx_id):
         # chop off the Mode/PID bytes based on the mode number
         mode = message.data_bytes[0]
         if mode == 0x43:
-            # GET_DTC requests (mode 03) do not have a PID byte
-            message.data_bytes = message.data_bytes[1:]
+
+            # fetch the DTC count, and use it as a length code
+            num_dtc_bytes = message.data_bytes[1] * 2
+
+            # skip the PID byte and the DTC count,
+            message.data_bytes = message.data_bytes[2:][:num_dtc_bytes]
+
         else:
             # handles cases when there is both a Mode and PID byte
             message.data_bytes = message.data_bytes[2:]

tests/test_protocol_can.py

@@ -133,11 +133,11 @@ def test_multi_line():
 		# MODE 03 COMMANDS (GET_DTC) RETURN NO PID BYTE
 
 		test_case = [
-			"7E8 10 20 43 00 01 02 03 04",
-			"7E8 21 05 06 07 08 09 0A 0B",
+			"7E8 10 20 43 04 00 01 02 03",
+			"7E8 21 04 05 06 07 08 09 0A",
 		]
 
-		correct_data = list(range(12))
+		correct_data = list(range(8))
 
 		r = p(test_case)
 		assert len(r) == 1