Fix #9: DTC/freeze-frame parsing (phantom codes, Mode 02, hex frame index)

- parse_dtcs CAN branch is now message-aware: each ECU reply '<svc> <count>
  <pairs>' has its header stripped per-message, instead of flattening all lines
  and stripping svc+count once. With multiple ECUs the old code ate the second
  header as a DTC pair -> phantom codes. Critically, it does NOT blind-scan for
  svc (0x43 is a legal DTC first byte: C03xx) — a numbered ISO-TP continuation
  is distinguished by its 'N:' frame-index prefix, not by value.
- _line_bytes strips hex frame indices A:-F: (ISO-TP index cycles 0-F), not just
  0-9, so consecutive frames past the 10th aren't dropped.
- read_freeze_frame sends the correct '020200' (svc 02, PID 02, frame 00) and
  skips SID+PID+frame (+3), fixing the off-by-one that mis-read the freeze DTC.
- tests/test_dtc_parse.py: single-frame, multi-ECU (no phantom), numbered
  multiframe with a real C03xx continuation, hex index, non-CAN legacy.

Closes #9

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016yT89n4zR4qbrySoSiEyZs

obdcore/link.py

@@ -23,11 +23,14 @@ def decode_dtc(b1, b2):
     return f"{_LETTER[(b1 >> 6) & 3]}{(b1 >> 4) & 3}{b1 & 0xF:X}{b2:02X}"
 
 
+_HEX = "0123456789ABCDEFabcdef"
+
+
 def _line_bytes(ln):
     ln = ln.replace(" ", "")
-    if len(ln) >= 2 and ln[1] == ":" and ln[0] in "0123456789":
-        ln = ln[2:]                         # drop CAN multiframe index "N:"
-    if not ln or any(c not in "0123456789ABCDEFabcdef" for c in ln):
+    if len(ln) >= 2 and ln[1] == ":" and ln[0] in _HEX:
+        ln = ln[2:]                         # drop CAN multiframe index "N:" (0-F, cycles)
+    if not ln or any(c not in _HEX for c in ln):
         return []
     return [int(ln[i:i + 2], 16) for i in range(0, len(ln) - 1, 2)]
 
@@ -35,11 +38,24 @@ def _line_bytes(ln):
 def parse_dtcs(lines, svc, is_can):
     pairs = []
     if is_can:
-        data = [b for ln in lines for b in _line_bytes(ln)]
-        if svc in data:
-            data = data[data.index(svc) + 1:]
-            data = data[1:] if data else data
-        pairs = data
+        # Message-aware: multiple ECUs each reply "<svc> <count> <pairs...>", and
+        # a DTC's own first byte can equal svc (0x43 == C03xx), so we must NOT
+        # blind-scan the flattened stream for svc. A line whose payload starts
+        # with svc begins a new ECU message (drop svc + count byte); an ISO-TP
+        # numbered continuation "N:" (N>=1) appends raw pairs to the current one.
+        started = False
+        for ln in lines:
+            raw = ln.replace(" ", "")
+            cont = len(raw) >= 2 and raw[1] == ":" and raw[0] in _HEX and raw[0] != "0"
+            b = _line_bytes(ln)
+            if not b:
+                continue
+            if not cont and b[0] == svc:      # header of an ECU message: svc + count
+                pairs.extend(b[2:])
+                started = True
+            elif started:                     # continuation / pairs of current message
+                pairs.extend(b)
+            # else: bytes before any header (ISO-TP length line, stray) -> ignore
     else:
         for ln in lines:
             data = _line_bytes(ln)
@@ -211,9 +227,9 @@ class ElmLink:
         """Mode 02: the DTC that set the freeze frame + the standard PID snapshot."""
         from . import obdservices as svc
         out = {"dtc": None, "values": []}
-        d = self._bytes(self.cmd("0202", timeout=timeout))
+        d = self._bytes(self.cmd("020200", timeout=timeout))   # svc 02, PID 02, frame 00
         if 0x42 in d:
-            r = d[d.index(0x42) + 2:]            # after '42 02'
+            r = d[d.index(0x42) + 3:]            # skip 42 (SID) 02 (PID) 00 (frame)
             if len(r) >= 2 and (r[0] or r[1]):
                 out["dtc"] = decode_dtc(r[0], r[1])
         for name, pid, nbytes, dec, unit in svc.FREEZE_PIDS: